THE WESTERN LAURENTIAN CONTINENTAL MARGIN IN THE CANADIAN CORDILLERA (Invited Presentation)

The western Laurentian margin formed after rifting that partially accommodated the Ediacaran to Cambrian breakup of supercontinent Rodinia. The rifted and subsequent passive margin are especially well-preserved in the Canadian Cordillera (Yukon and British Columbia). Evidence for a diachronous breakup, older (570-520 Ma) in the south and younger (500 Ma) in the north, includes development of breakup unconformities and rift-related volcanism (ca. 570 Ma and 501-497 Ma) followed by thermal subsidence. Rift-related sedimentation resulted in thick accumulations of immature clastic rocks including turbidite successions of the Windermere Supergroup and Hyland Group. A series of rift-transfer faults resulted in segmentation of the lower Paleozoic margin that have influenced the architecture of the continental margin, manifested as offsets of platform and basin successions. Passive margin sedimentation persisted from the Cambrian to the Lower Devonian and is characterized by shallow-water carbonate and sandstone in platformal regions and deep-water shale, chert and sandstone in basinal successions. Post-rift, alkaline mafic magmatism occurs throughout the lower Paleozoic successions with intervals dated by CA-TIMS at 493-491 Ma, 486-484 Ma and 453-445 Ma. Post-rift magmatism is localized near transitions in sedimentary facies and is interpreted to have been accommodated by reactivation of inherited rift structures, including transfer faults. Passive margin sedimentation was interrupted in the Middle to Late Devonian with initiation of subduction, onset of arc magmatism in the Yukon-Tanana terrane and back-arc extension associated with opening of the Slide Mountain Ocean. This is reflected in the continental margin by renewed extension, coarse clastic sedimentation and sporadic magmatism in the Earn Group, with episodes dated by CA-TIMS at 378 Ma and 365-363 Ma. By the Middle to Late Mississippian, passive margin sedimentation resumed along western Laurentia, until the influx of westerly derived detritus in Middle to Late Triassic clastic rocks that heralded the subsequent development of the Cordilleran orogen. The Neoproterozoic architecture influenced the Paleozoic history of the margin and the development of the Mesozoic orogenic belt.