Consequences of the Cretaceous west-dipping subduction in western North America

Abstract

The westward growth of the North American Plate through amalgamation of the Cordillera terranes and North America Craton is a fundamental issue in tectonics. There is mounting evidence for Cretaceous westward subduction below the east side of the Cordillera, leading to terminal Cordillera-Craton collision; this contrasts with the long-held view of persistent east-dipping subduction on the west side. One key observation is the Omineca Magmatic Belt (OMB, 120-90 Ma) on the eastern side of the Cordillera. The OMB is associated with I-type plutons prior to 110 Ma, followed by an evolution to S-type granites. This is consistent with arc magmatism, where the mantle wedge progressively becomes contaminated with sediments as the Craton margin approached the plate margin.We first examine the Craton response to subduction. As the strong Craton entered the trench, it experienced flexure, creating a topographic bulge. Flexural calculations for a 120 km thick elastic plate show that flexure would have induced tensile stresses in the deep Craton lithosphere 400-1200 km east of the subduction margin. The location of tensile stresses, the origination of the subduction margin, and the timing of Craton flexure correlate with the Central Cretaceous Kimberlite Corridor (CCKC, 115-92 Ma). This provides a novel explanation for kimberlite magmatism. Next, we characterize additional observational constraints on Cretaceous westward subduction. First, we use 2D upper mantle scale numerical models to investigate the dynamics of subduction and Cordillera-Craton collision. Models show that following collision, the subducted oceanic plate detached. Break-off was accompanied a pulse of surface uplift, with greater uplift for a stronger cratonic lithosphere. Future work will involve how subduction and break-off would be recorded in the OMB. Our studies constrain the feasibility of west-dipping subduction under the Cordillera during the Cretaceous.