Abstract
Abstract The Intermontane-Insular terrane boundary stretches over 2000 kilometers from British Columbia to Alaska in the western Cordillera. Juxtaposed between these terranes is a series of Jura-Cretaceous basinal and arc assemblages that record a complicated and contested tectonic evolution related to the Mesozoic-Paleocene accretionary history of northwestern North America. In southwest Yukon, west-verging thrust faults facilitated structural stacking of the Yukon-Tanana terrane over these basinal assemblages, including the Early Cretaceous Blanchard River assemblage. These previously undated compressional structures are thought to be related to the final collapse of the Jura-Cretaceous basins and the tectonic burial of the Blanchard River assemblage resulting in amphibolite facies metamorphism. New in situ U-Th-Pb monazite ages record at least three tectonic events: (1) the tectonic burial of the Blanchard River assemblage to amphibolite facies conditions between 83 and 76 Ma; (2) peak burial was followed by regional exhumation at ca. 70-68 Ma; and (3) intense heating and ca. 63-61 Ma low-pressure contact metamorphism attributed to the intrusion of the voluminous Ruby Range suite, which is part of the northern Coast Mountains batholith. The tectonometamorphic evolution recorded in the Blanchard River assemblage can be correlated to tectonism within southwest Yukon and along the length of the Insular-Intermontane boundary from western British Columbia through southwestern Yukon and Alaska. In southwest Yukon, these results suggest an asymmetric final collapse of Jura-Cretaceous basins during the Late Cretaceous, which relates to the terminal accretion of the Insular terranes as they moved northward.
Highlights
Since Devonian time, convergent margin tectonism has defined the architecture of the western margin of Laurentia, resulting in the accretion of allochthonous terranes progressing from the late Paleozoic-early Mesozoic into the early Cenozoic [1,2,3,4,5,6,7,8,9,10]
Metamorphic, structural, and petrochronological results from this study provide important new information to reconstruct the tectonometamorphic evolution for the Blanchard River assemblage, and by corollary new timing constraints related to metamorphism associated with terminal accretion of the Insular terranes in southwest Yukon
Following ca. 126 mean ages (Ma) deposition of the Blanchard River assemblage and up to the Paleogene intrusion of the Ruby Range suite (6451 Ma), there are at least three distinct tectonic events that affected the region including (1) overthrusting of YukonTanana terrane resulting in tectonic thickening and amphibolite facies metamorphism (M1C-DT), (2) decompression resulting in the growth of M2C minerals, followed by DT+1 deformation, and (3) intrusion of the Ruby Range suite resulting in the replacement of previous metamorphic mineral assemblage with high-temperature, low-pressure M3C minerals
Summary
Since Devonian time, convergent margin tectonism has defined the architecture of the western margin of Laurentia, resulting in the accretion of allochthonous terranes progressing from the late Paleozoic-early Mesozoic into the early Cenozoic [1,2,3,4,5,6,7,8,9,10]. Two principal and contrasting models exist to reconcile the accretionary history of the Insular terranes, which include an initial Middle Jurassic [11, 12, 16,17,18] and final (terminal) mid- to Late Cretaceous accretion and/or collision [2, 14, 15, 19,20,21,22,23,24,25], or a single accretionary event from the Late Jurassic into the Cretaceous [26, 27]. Both models do agree that components of the Insular terranes did not terminally accrete until mid- to Late Cretaceous time.
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call