Investigating the Effect of Mantle Flow and Viscosity Structure on Surface Velocities in Alaska Using 3‐D Geodynamic Models

Abstract

We utilize 3‐D finite element geodynamic models, incorporating long‐term kinematic estimates of upper plate motion, to better understand the roles that viscosity structure and mantle tractions play in generating plate motions and continental interior deformation in Alaska. Surface deformation in the Pacific‐North American plate boundary zone in Alaska and northwest Canada is strongly influenced by the complex interactions between flat‐slab subduction, gravitational collapse and mantle tractions. Predictions of long‐term tectonic block motion derived from recent Global Positioning System datasets (GPS) reveal surface motions atypical of other continental convergent plate boundary zones. Specifically, in northern and northwestern Alaska, southeastward motion is observed directed back toward the plate boundary. Geodynamic models that incorporate a southeastward directed long‐wavelength mantle traction of ∼2.5–3.8 MPa best replicate surface velocities in Alaska. These mantle tractions, in conjunction with the collision of the Yakutat microplate, appear to drive the uplift and deformation in the Mackenzie Mountains. Furthermore, the extent of the northward motion in southern and central Alaska is controlled by the location of the leading edge of the Yakutat flat slab.

New Insights Into the Active Tectonics of the Northern Canadian Cordillera From an Enhanced Earthquake Catalog

“…The trend of moderate‐magnitude earthquakes continues further south, generally occurring southwest of the Snake River fault and focal mechanisms suggest a combination of thrusting and dextral strike‐slip motion. The structures in the Richardson Mountains that we identify as displaying high levels of seismicity are all aligned essentially north‐south, subparallel to the orientation of the inferred mantle traction field in the north of our study region (i.e., McConeghy et al., 2022).…”

“…Recent work has highlighted the importance of the additional effect of southward‐oriented mantle traction forces (i.e., Finzel et al., 2015) that extend through the Richardson mountains and are deflected by subducting Yakutat lithosphere in central Alaska. This deflection is modeled to re‐orient toward the northeast in the NCC (McConeghy et al., 2022). In this section, we correlate the observed seismicity with previously mapped structures to better describe how these regional stresses relate to tectonically active faults.…”

“…These traction forces are thought to be the result of global mantle flow that is oriented north-south through Arctic Canada and Alaska and is deflected by subducting Yakutat lithosphere beneath Alaska. McConeghy et al (2022) presented a geodynamic model for surface displacements based on mantle tractions that showed that uplift in the Mackenzie Mountains does not require strain transfer along a lower crustal detachment, and that instead the flow of mantle material away from the Yakutat collision zone possibly interacting with the north-south oriented flow originating from the Arctic alone is enough to produce thrust earthquakes in the Mackenzie Mountains.…”

Variations in Lithospheric Thickness Across the Denali Fault and in Northern Alaska

“…While higher Mg# or other variations in bulk chemistry (e.g., Dalton & Faul, 2010; Garber et al., 2018) may also contribute to increasing Vs, the inference of lower mantle temperature is corroborated by low heat flow relative to the rest of Alaska in the northern Arctic Alaska terrane (with the exception of its northernmost tip which is not included in our model) (Batir et al., 2016). The higher viscosities expected for lower temperatures, in combination with distance from subduction in the south, help to explain the low degree of deformation internal to the northern Arctic Alaska terrane shown by geodetic and fault slip data (e.g., Elliott & Freymueller, 2020; Finzel et al., 2015; McConeghy et al., 2022), in contrast to much higher strain rates in the thinner and more deformable lithosphere to the south.…”

Synthesis of the Seismic Structure of the Greater Alaska Region: Continental Lithosphere

“…The average crustal velocity profile in the C6 domain (which reflects continental crust over the subducting plate) provides new constraints on the buoyancy of the Yakutat crust. These constraints will be useful for lithospheric-scale (e.g., Finzel et al, 2015;McConeghy, Flesch, & Elliott, 2022) and mantle-scale (e.g., Jadamec & Billen, 2010, 2012Jadamec et al, 2013;Haynie & Jadamec, 2017) geodynamical models of subduction in Alaska and its impact on the overriding continental lithosphere. The crustal seismic velocities and thickness constraints synthesized in this study could also help to better design representative models of upper plate dynamics (Torne et al, 2019) and models of plateau subduction to examine the effects of plateau subduction-collision on long-term plate boundary evolution (e.g., Koons et -27-manuscript submitted to AGU Monograph al., 2010;Haynie, 2019;Moresi et al, 2014) and the role of eclogitization of the subducting plateau with depth (Arrial & Billen, 2013).…”