Is There a Speed Limit for the Thermal Steady‐State Assumption in Continental Rifts?. (16th March 2021)

• Record Type: Journal Article
• Authors: Heckenbach, Esther L.; Brune, Sascha; Glerum, Anne C.; Bott, Judith
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 3(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-03-16
• Subjects: basin analysis -- geodynamics -- numerical modeling -- rifting -- thermal modeling; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2020GC009577
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The lithosphere is often assumed to reside in a thermal steady‐state when quantitatively describing the temperature distribution in continental interiors and sedimentary basins, but also at active plate boundaries. Here, we investigate the applicability limit of this assumption at slowly deforming continental rifts. To this aim, we assess the tectonic thermal imprint in numerical experiments that cover a range of realistic rift configurations. For each model scenario, the deviation from thermal equilibrium is evaluated. This is done by comparing the transient temperature field of every model to a corresponding steady‐state model with an identical structural configuration. We find that the validity of the thermal steady‐state assumption strongly depends on rift type, divergence velocity, sampling location, and depth within the rift. Maximum differences between transient and steady‐state models occur in narrow rifts, at the rift sides, and if the extension rate exceeds 0.5–2 mm/a. Wide rifts, however, reside close to thermal steady‐state even for high extension velocities. The transient imprint of rifting appears to be overall negligible for shallow isotherms with a temperature less than 100°C. Contrarily, a steady‐state treatment of deep crustal isotherms leads to an underestimation of crustal temperatures, especially for narrow rift settings. Thus, not only relatively fast rifts like the Gulf of Corinth, Red Sea, and Main Ethiopian Rift, but even slow rifts like the Kenya Rift, Rhine Graben, and Rio Grande Rift must be expected to feature a pronounced transient component in the temperature field and to therefore violate the thermal steady‐state assumption for deeper crustal isotherms. Plain Language Summary: Temperature distribution is a key factor when studying Earth's interior. Here, we quantify the influence of rift velocity on temperature distribution with numerical simulations. As a continent begins to split, forming a rift, hot material beneath the rift center moves upwards increasing the temperatures at shallow crustal depth. However, simple thermal models often assume an equilibrated, constant temperature field. To evaluate tectonically induced changes in temperatures, we compare lithosphere‐scale dynamic models to models with the same material configuration but with a steady‐state temperature distribution and no deformation. We find that the latter approach well represents locations outside the rift valley and shallow crustal depths where comparably low temperatures prevail. Contrarily, at the sides of the rift valley, or for deeper‐lying isotherms like 400°C, the assumption of an equilibrated thermal field leads to an underestimation of crustal temperatures. Furthermore, we show that temperatures in narrow rifts like the Kenya Rift and the Rhine Graben are more strongly modified by tectonic movements than temperatures in wide rifts, like the Basin and Range Province. Finally, we conclude that the assumption of constant temperatures is only valid for wide rifts and very slowly extending narrow rifts with a speed limit of <0.5–2 mm/a. Key Points: We use numerical forward modeling to quantify the transient thermal imprint of continental extension The validity of the thermal steady‐state assumption in narrow rifts is limited to extension velocities less than 0.5–2 mm/a Transient thermal effects are particularly significant for deep isotherms, narrow rifts, and beneath the rift shoulders