Reconciling lithospheric rheology between laboratory experiments, field observations and different tectonic settings

Abstract

Summary Recent modeling studies have shown that laboratory-derived rheology is too strong to reproduce observations of flexure at the Hawaiian Islands, while the same rheology appears consistent with outer rise—trench flexure at circum-Pacific subduction zones. Collectively, these results indicate that the rheology of an oceanic plate boundary is stronger than that of its interior, which, if correct, presents a challenge to understanding the formation of trenches and subduction initiation. To understand this dilemma, we first investigate laboratory-derived rheology using fully dynamic viscoelastic loading models and find that it is too strong to reproduce the observationally inferred elastic thickness, Te, at most plate interior settings. The Te can, however, be explained if the yield stress of low-temperature plasticity is significantly reduced, for example, by reducing the activation energy from 320 kJ/mol, as in Mei et al. (2010), to 190 kJ/mol as was required by previous studies of the Hawaiian Islands, implying that the lithosphere beneath Hawaii is not anomalous. Second, we test the accuracy of the modeling methods used to constrain the rheology of subducting lithosphere, including the Yield Stress Envelope (YSE) method, and the Broken Elastic Plate Model (BEPM). We show the YSE method accurately reproduces the model Te to within ∼10 per cent error with only modest sensitivity to the assumed strain rate and curvature. Finally, we show that the response of a continuous plate is significantly enhanced when a free edge is introduced at or near an edge load, as in the BEPM, and is sensitive to the degree of viscous coupling at the free edge. Since subducting lithosphere is continuous and generally mechanically coupled to a sinking slab, the BEPM may falsely introduce a weakness and hence overestimate Te at a trench because of tradeoff. This could explain the results of recent modeling studies that suggest the rheology of subducting oceanic plate is stronger than that of its interior. However, further studies using more advanced thermal and mechanical models will be required in the future in order to quantify this.

Plate bending earthquakes and the strength distribution of the lithosphere

“…, the values for old lithosphere lie within the range of intermediate strength models. Therefore, the 'conundrum' of strong plates at subduction zones and weak plates around seamounts, may be less acute than assumed in recent studies Bellas et al (2022).…”

“…1. We note that a range of other studies have converged on values of 0.3 for the effective friction coefficient, in both subduction zone in seamount settings Pleus et al, 2020;Bellas et al, 2022). A useful reference value that represents the strength of this bestfitting model is that it exhibits a typical saturation moment close to 1.5 ×10 17 N, across the range of old lithosphere (> 80 Myr); it lies close to the middle of what we call the intermediate strength region.…”

“…Many of the factors that enter the YSE calculation process are subject to debate within geoscience disciplines. For instance, if either net axial force (see 3.4) due to slab pull is significant (> typical ridge push magnitudes), or if outer-rise normal faults have low friction coefficient (< 0.3) -as some evidence suggests -the ∼ 30 km neutral plane observations are difficult to reconcile, unless differential stress levels in the ∼ 30-60 km region to are significantly reduced, at least relative to stronger LTP strength predictions (e.g., , in line with conclusions that have emerged from recent studies of seamount flexure (Zhong & Watts, 2013;Bellas et al, 2022).…”

“…Data from Fig. 10 Bellas et al (2022). One of the confounding factors for seamount flexure is that thermal perturbation due to hotspot activity may reduce the apparent strength of the lithosphere (e.g., Pleus et al, 2020), although Bellas et al (2022) have argued that this is not capable of explaining seamount flexure generally.…”

“…10 Bellas et al (2022). One of the confounding factors for seamount flexure is that thermal perturbation due to hotspot activity may reduce the apparent strength of the lithosphere (e.g., Pleus et al, 2020), although Bellas et al (2022) have argued that this is not capable of explaining seamount flexure generally. When thermal perturbation is ignored, strength models with a friction coefficient of 0.3 have been inferred by several studies Bellas & Zhong (2021); Pleus et al (2020).…”