Lithospheric S Wave Velocity Variations Beneath the Mackenzie Mountains and Northern Canadian Cordillera

Abstract

The Mackenzie Mountains (MMs) in the Yukon and Northwest Territories, Canada, are an enigmatic mountain range. They are currently uplifting (Leonard et al., 2008, http://https//doi.org/10.1029/2007JB005456), yet are about 700 km from the nearest plate boundary. Their arcuate shape is distinct and extends over 100 km eastward from the general trend of the Northern Canadian Cordillera. To better assess the cause and conditions of the current uplift, we processed ambient seismic noise data from a linear array of broadband seismographs crossing the mountains, along with other regional seismic stations, to estimate Rayleigh wave phase velocities between 6 and 40 s periods. From this, we estimated phase velocity dispersion and performed a tomographic inversion to estimate VS. Tomography reveals a low‐velocity structure that extends upward from the base of the ∼50–66 km thick lithosphere to the upper crust, and we hypothesize that inferred low density and low rigidity associated with the VS anomaly localizes the ongoing uplift and thrust‐dominated seismicity of the MMs. Additionally, we find relatively low crustal velocities that extend to the west of the MMs, suggesting that strain transfer from the Gulf of Alaska plate boundary plays a driving role as the crust translates to the northeast and buckles up against the craton consistent with the orogenic float hypothesis of Mazzotti and Hyndman (2002, http://https//doi.org/10.1130/0091-7613(2002)030%3C0495:YCASTA%3E2.0.CO;2). Finally, we observe lithospheric azimuthal anisotropy with an NW‐SE fast direction. This is nearly orthogonal to teleseismic shear wave splitting measurements in the central MMs, and suggests that asthenosphere flow and lithospheric strain are not aligned in this region.

A New P‐Wave Tomographic Model (CAP22) for North America: Implications for the Subduction and Cratonic Metasomatic Modification History of Western Canada and Alaska

“…In the south, the un‐modified, depleted, fast wavespeed Laurentian lithosphere provides a strong backstop to deformation and limits the width of Cordilleran lithosphere. This is broadly consistent with inferences from ambient noise tomography from the Mackenzie Mountains indicating low lithospheric S‐wavespeeds, and inferred rheological weakness, beneath the active Mackenzie thrust (Schutt et al., 2023), and its buttressing to the north and south by higher velocity and inferred stronger lithosphere (Estève et al., 2020). Together, these observations suggest that both tectonic inheritance from asymmetric rifting (Thomas, 2006) and lithospheric mantle strength variations control orogenic styles along the CCD.…”

Variations of Crustal Thickness in Alaska and Northwestern Canada: Implications for Crustal Modification and Accretion of Tectonic Units

“…The properties of the continental lithosphere vary significantly within the interior of northwestern Canada. Many tomographic models have revealed two high‐velocity anomalies in the upper mantle beneath the northern Canadian Cordillera (Estève et al., 2020; McLellan et al., 2018; Schutt et al., 2023). The first anomaly is located beneath the northeastern NAM, which correlates with the Mackenzie craton.…”