Deep vs. shallow controlling factors of the crustal thermal field – insights from 3D modelling of theBeaufort‐MackenzieBasin (ArcticCanada)

Abstract

Significant lateral variations in observed temperatures in the Beaufort-Mackenzie Basin raise the question on the temperature-controlling factors. Based on the structural configuration of the sediments and underlying crust in the area, we calculate the steady-state 3D conductive thermal field. Integrated data include the base of the relic permafrost layer representing the 0°C-isotherm, public-domain temperature data (from 227 wells) and thermal conductivity data. For >75% of the wells the predicted temperatures deviate by <10 K from the observed temperatures, which validates the overall model setup and adopted thermal properties. One important trend reproduced by the model is a decrease in temperatures from the western to the eastern basin. While in the west, a maximum temperature of 185°C is reached at 5000 m below sea level, in the east the maximum temperature is 138°C. The main cause for this pattern lies in lateral variations in thermal conductivity indicating differences in the shale and sand contents of the different juxtaposed sedimentary units. North-tosouth temperature trends reveal the superposition of deep and shallow effects. At the southern margin, where the insulating effect of the low-conductive sediments is missing, temperatures are lowest. Farther north, where the sub-sedimentary continental crust is thick enough to produce considerable heat and a thick pile of sediments efficiently stores heat, temperatures tend to be highest. Temperatures decrease again towards the northernmost distal parts of the basin, where thinned continental and oceanic crust produce less radiogenic heat. Wells with larger deviations of the purely conductive model from the temperature observations (>15 K at 10% of the wells) and their basin-wide pattern of misfit tendency (too cold vs. too warm temperature predictions) point to a locally restricted coupling of heat transport to groundwater flow.

Heat Flow on the U.S. Beaufort Margin, Arctic Ocean: Implications for Ocean Warming, Methane Hydrate Stability, and Regional Tectonics

“…The high heat flow at depths of 1,000–1,200 mbsl may therefore be the combined effect of higher radiogenic heat production and higher temperatures in the thick, lower conductivity sediment packages blanketing deeper crust at the continent‐ocean transition. This hypothesis is similar to what is suggested at the continent‐ocean transition to the west in the Beaufort‐Mackenzie Basin of Canada (e.g., Sippel et al, 2015), although the heat flow values observed on the Canada Margin are, on average, a factor of 1.5–3 lower. Sediment thickness is greatest at the U.S. Beaufort Margin ocean‐continent transition, where it likely exceeds 14 km (Mosher et al, 2012; Chian et al, 2016).…”

“…Nonetheless, we cannot rule out other hypotheses. For example, the higher heat flow observed in water depths of 1,000–1,200 mbsf may be, in part, associated with deeper, tectonic phenomena, such as reactivation of the failed Dinkum Graben that helped establish the margin (Grantz & May, 1982), or thermal blanketing (e.g., Sippel et al, 2015). Geologic, seismic, and structural interpretations for the Beaufort Margin suggest large, potentially active growth faults exist along the shelf and upper margin with possible extension in the west and thrusting in the east (e.g., Grantz et al, 1979; Haimila et al, 1990; Hasegawa et al, 1979; Houseknecht & Bird, 2011; Houseknecht et al, 2012).…”

Surface heat flow measurements from the East Siberian continental slope and southern Lomonosov Ridge, Arctic Ocean

“…In the Amerasian Basin, reduced surface heat flow toward the outer continental slope of the Beaufort Mackenzie Basin was reported by Jones et al [1990], and attributed to thinner crust and less radiogenic heat production. Recently, 3-D thermal modeling of the Beaufort Mackenzie Basin, constrained by temperature data from over 200 deep boreholes, also identified a decrease in thermal gradients as the crustal thickness decreases toward the ocean-continent transition [Sippel et al, 2015]. Regional and local variations in the heat flow data presented for the East Siberian continental slope are also likely driven by changes in crustal thickness and composition, as well as radiogenic inputs from the highly variable sedimentary cover (ranging from meters to kilometers thick).…”

Variability of the geothermal gradient across two differently aged magma-rich continental rifted margins of the Atlantic Ocean: the Southwest African and the Norwegian margins

“…This difficulty largely arises from the variability of the lithosphere in terms of structure and composition, parameters that are a product of the tectonic setting and evolution of the location of interest. One well-established strategy to investigate the present-day thermal field of a certain area is to integrate existing geophysical and geological data into 3-D structural models that provide the basis for numerical modeling, which simulates heat transport processes after setting boundary conditions and thermal properties according to the geological structure (e.g., Scheck-Wenderoth and Lamarche, 2005;Noack et al, 2013;Scheck-Wenderoth et al, 2014;Sippel et al, 2015;Balling et al, 2016).…”