The Thermal Regime of NW Canada and Alaska, and Tectonic and Seismicity Consequences

Abstract

NW Canada and Alaska are the continuation of the North American Cordillera through Mexico, western USA and western Canada. I show that they have similar thermal regimes and thermal control of tectonics and seismicity. I first summarize the multiple constraints to crust and upper mantle temperatures and then discuss some consequences. There are bimodal crust and upper mantle temperatures characteristic of most subduction zones: cool forearc, uniformly hot backarc (Yukon Composite Terrane to Southern Brooks Range, and Mackenzie Mountains), and stable cratonic backstops (Arctic Alaska Terrane and Canadian Shield). The main constraints are as follows: (a) Heat flow measurements, (b) Temperature‐dependent upper mantle velocities and seismic attenuation, (c) Temperature‐dependent topographic elevations; thermal isostasy, (d) Depth and temperature of the seismic lithosphere‐asthenosphere boundary (LAB), (e) Origin temperature and depth of craton kimberlite xenoliths, (f) Geochemically inferred source temperature and depth of recent volcanic rocks. (g) Depth to the magnetic Curie temperature, (h) Depth extent of seismicity. The backarc lithosphere is thin, LAB at 50–85 km and ∼1,350°C ± 25°C. Moho temperatures at 35 km are 850°C ± 100°C compared to cool cratonic areas of 400°C–500°C. The consequences include the following: (a) Thin and weak backarc lithosphere that accommodates pervasive tectonic deformation indicated by wide‐spread seismicity and GPS‐defined motions, in contrast to the stable cratonic regions; (b) Weak backarc lower crust that flattens the Moho and allows detachment and thrusting of the upper crust over the cold strong Arctic Alaska Terrane and Canadian Shield. This article provides a model for how to estimate deep temperatures from multiple constraints.

Earthquake Relocations Delineate a Discrete Fault Network and Deformation Corridor Throughout Southeast Alaska and Southwest Yukon

“…Seismicity largely occurs in the brittle regime of the Earth's crust and, therefore, estimates of seismogenic thickness can be used to define the brittle‐plastic transition (Furlong & Atkinson, 1993; Hyndman, 2023; Miller & Furlong, 1988). This allows us to place constraints on the thermal structure of the crust inboard of the Yakutat indentor corner as the brittle‐plastic transition is a thermal rheological transition which occurs at approximately 350°C for quartzo‐felspathic continental crusts (Hyndman et al., 1997; Sibson, 1982).…”