Late Cambrian and Ordovician Alkalic Magmatism Along the Dawson Fault, Yukon: Evidence for Multiple Reactivations of a Rift‐Transfer Fault in the Northern Canadian Cordillera.

Lower Paleozoic continental margin rocks of the North American Cordillera, from Yukon to Nevada, include coeval platformal carbonate and basinal clastic strata that are offset along rift‐transfer faults, including the Liard, St. Mary‐Moyie, and Snake River structures. The Dawson fault is a prominent east‐west‐striking structure in central Yukon that is interpreted herein to have been active as a rift‐transfer fault by late Cambrian time. This hypothesis is supported by new zircon U‐Pb dates that range from 501.98 ± 0.17 Ma to 497.57 ± 0.70 Ma from alkaline mafic volcanic rocks concentrated along the Dawson fault. The development of a sub‐Jiangshanian unconformity immediately post‐dates this alkaline magmatism and indicates that final continental breakup and establishment of the northern Cordilleran margin occurred by the late Miaolinginan. Alkaline magmatism caused by local decompression partial melting of the mantle may have been triggered by the release of in‐plane tensile stresses during lithospheric rupture and edge‐driven mantle convection. Upper Ordovician alkaline mafic volcanic and plutonic rocks that occur along a northwest‐southeast striking segment of the Dawson fault erupted ∼50 Myr after breakup and represent an example of post‐rift magmatism along a rift‐transfer fault. New bedrock mapping, and geochronological, paleontological, and petrological results from Upper Ordovician rocks indicate that there was localized basin development and punctuated volcanism along the Dawson fault from 453 to 447 Ma. Late Ordovician extension and post‐breakup magmatism in central Yukon is compatible with dextral strike‐slip reactivation of the Dawson rift‐transfer fault associated with counterclockwise rotation of Laurentia. Plain Language Summary: Sedimentary rocks that formed in shallow‐ and deep‐water ocean settings along the western edge of the ancient North American craton around 500 to 370 million years ago are offset by margin–perpendicular faults. These faults formed when continental lithosphere stretched and eventually broke apart between 750 and 500 million years ago and are called rift‐transfer faults. We propose that the Dawson fault, an east‐west oriented fault in central Yukon, Canada, was active as a rift‐transfer fault around 500 million years ago. At this time, lava poured out onto the ocean floor along the Dawson fault and our study precisely dates volcanism between 501 and 497 million years. We also studied younger volcanic rocks that occur along the Dawson fault and link these eruptions to its reactivation around 453–445 million years ago. This younger volcanism is associated with stretching and thinning of the continental crust and underlying upper mantle, which had two main effects: (a) The mantle partially melted to form magma that formed underwater volcanoes; and (b) the surface of the Earth subsided and formed marine basins. We propose that these processes occurred together along the Dawson fault and resulted in the deposition of 453–445 million year old volcanic and deep water sedimentary rocks. Key Points: Syn‐rift to breakup‐related magmatism along the Cordilleran margin in Yukon, Canada, occurred from 502 to 497 MaThe Dawson fault is a rift‐transfer fault that was active during final rifting and early Paleozoic establishment of the western Laurentian marginPost‐rift magmatism occurred from 453 to 445 Ma and was linked to strike‐slip reactivation of the Dawson fault [ABSTRACT FROM AUTHOR]