Deformation at the eastern margin of the Northern Canadian Cordillera: Potentially related to opening of the North Atlantic

The Northern Canadian Cordillera (NCC) comprises the Mackenzie Mountains, which are characterized by earthquakes occurring ~1,000 km east of the western North American margin. However, no recognized convergence has occurred in this inboard region since the Mesozoic to early Cenozoic formation of the Cordillera. This lack of an obvious driver for the modern NCC deformation has generated considerable debate and various geodynamic models. We show here thermal histories derived from (U‐Th‐Sm)/He data that are interpreted to indicate reactivated deformation and formation of the eastern deformation front beginning ~30 Ma. At that time the western margin of North America was mainly a transform boundary, which typically transmits very limited amount of stress into the continent. Along the northeastern margin of North America, however, the North Atlantic was opening and may have caused horizontal forces that drove deformation far to the west where the rigid craton encountered the weak Cordillera.


| METHODS AND RE SULTS
The cooling history of rocks can be revealed by low-temperature thermochronology. Analysing apatite and zircon with (U-Th-Sm)/He thermochronology quantifies cooling through temperatures of 40-

Pacific Plate
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Apatite and zircon grains were separated using standard separation techniques. Individual grains were examined and selected under a stereomicroscope, measured for grain size dimension for alpha-ejection correction (Farley, Wolf, & Silver, 1996), and packed in niobium packages.
Packages were sent to the University of Arizona at Tucson for analysis of He, U, Th and Sm. We aimed to analyse five apatite and three zircon grains per sample. Single grain data are available in Tables S1 and S2.
In order to better interpret the thermochronology data we conducted thermal history modelling using the program HeFTy v1.9.3 (Ketcham, 2005). We used the radiation damage accumulation and annealing mod- The most significant outcome of our study is the clear evidence of a third cooling phase that is interpreted as recoding deformation and exhumation during Oligocene-early Miocene (33-20 Ma; Figure 3b).
Thermal history modelling of our data suggest cooling from 120-80°C

This model requires high elevations in the St. Elias Mountains by
Oligocene time in order to produce the gravitational potential to cause the upper crust to flow northeastward. The St. Elias Mountains have been extensively studied using thermochronology, and collectively these data support rapid exhumation and inferred mountain building beginning 15-10 Ma (e.g. Enkelmann et al., 2017;O'Sullivan & Currie, 1996). We therefore rule out this model as a feasible driving mechanism for early Oligocene-early Miocene deformation at the eastern NCC.   (Gaina, Gernigon, & Ball, 2009). Ocean floor studies reveal that separation between Greenland and Eurasia developed in a series of failed rifts, plate boundary relocations and microcontinent development (Ellis & Stocker, 2014;Gaina et al., 2009). Oligocene-Miocene plate reorganization resulted in a link between the Reykjanes and Kolbeinsey ridges southeast of Greenland, which created the final break between Europe and North America and the emergence of Iceland (Ellis & Stocker, 2014). These reorganizational processes had a major influence on the motion of the North American plate, which rotated counterclockwise (from NW to SW direction) in the Oligocene (Gaina et al., 2009). From that time on, Greenland and North America moved together towards the west (Figure 4). Plate motion changed to eastward motion by 20 Ma, which may explain the observed decease of cooling and inferred deformation and rock exhumation at the eastern NCC. Today, North America is moving westward with high rates and may explain the ongoing deformation (Figures 2 and   4). Modern stress measurements reveal that the entirety of northern North America east of the Cordillera shows a NE-SW orientation of the maximum horizontal stresses, and that Cordilleran topography has a very limited affect on the eastern foreland stresses (Reiter, Heidbach, Haug, Ziegler, & Moeck, 2014). The correlation of the NE-SW orientation of maximum horizontal stresses with the orientation of the Mid-Atlantic ridge and the plate motion of North America (Henton, 2006) suggest crustal stress today is caused by ridge push and basal tractions from mantle convection (Coblentz & Richardson, 1995;Zoback et al., 1989).
We propose that Oligocene plate reorganization in the northwestern hemisphere initiated southwest-directed motion of cratonic