I have reviewed the revised version of the manuscript “The effect of low-viscosity sediments on the dynamics and accretionary style of subduction margins” presenting a suite of numerical subuction models investigating the influence of input sediment layer thickness and viscosity on subduction dynamics and wedge/interface properties.
While the authors have made revisions or fair rebuttal to certain comments of the initial two reviews (e.g. title, abstract, discussion, wedge angle vs. volume, new Figure 2, additional figures in the supplementary material), I find that the issue regarding designation and classification of “tectonic erosion” vs. “accretionary wedge” margins (details hereafter) still makes major revisions necessary before publication.
- Indeed, no actual tectonic erosion is observed in the presented models. The authors state in the results section that “Entrainment of sediments within upper plate material at the interface is indicative of some erosion of the upper plate” (line 253), but incorporating sediments to the upper plate is the opposite to upper plate erosion.The authors choice of labelling small accretionary wedge as a “tectonic erosive” margin thus seems impromper, and misleading comparibg to nature, where the inverse relationship between convergence rate and sediment thickness is observed (Supp. Fig. S2 vs. Fig. 4).
- The distinction between “accretionary” vs. “tectonic erosion” models only relies on diagnostics quantifying the degree of sediment accretion (wedge width, wedge angle), subduction dynamics (convergence and trench rate) and slab morphology (radius of curvature). For these diagnostics, there is no clear dichotomy in the models (cf Supp. Fig. S9) but rather a continuity from small wedges (low convergence velocity, small wedge angle and width, small radius of curvature associated to steeper slabs) to large wedges (vice versa).
- Hence I strongly advise against labelling the models as “tectonic erosion” in the results section. I suggest that the authors discuss how low-accretion margins could related to tectonic-erosive margins in the discussion, but present their results (Fig. 3-6) as a function of input model parameters (e.g. sediment thickness and viscosity). End-members simulations would thus become “low viscous coupling” (large thickness, low viscosity) vs. “high viscous coupling” (small thickness, large viscosity).
Other comments :
- Despite the zoom, velocity arrows are still illegible on the left and middle panel of Figure 2. I suggest that this Figure is rotated (landscape format) to enlarge the velocity field, referred to on lines 250 (“”the motion between subducting and upper plate in tectonic erosion margin is accommodated in the middle of the sediment layer, a region of high strain-rates) and 275 (“internal counter-clockwise flow inside accretionary wedges”). Another possibilité is to reduced the number of arrows and increase their scaled size.
- the criteria of “transient vs. steady-state” (line 227, 246, 306) cannot be used to discriminate between margins types, cf Supp. Fig. S9. The authors should directly refer to the continuous spread of wedge angle and width between high and low end-members (Fig. 4).
- An interesting result is the partitioning of weak interface material between subduction and wedge (Fig. 6). Even though the “sediment” layer in the models is not buoyant, I find the models relevant at first-order given that subduction interface is a melange of (lighter) sediments and (denser) metamorphic oceanic crust. However, Fig. 6 presents the relative volume fraction (in %) as a function of a dimensionless time, which makes it hard to directly compare absolute subducted volumetric flux rates (in m3/yr) between simulations (as noted by the authors lines 430-434). Maybe the authors could estimated an absolute value rather than a relative fraction (since initial sedimental thickness vary between simulations)
- I am not sure I follow the authors when they say that “The definition of subduction interface in numerical models could also be relaxed” (line 574). Indeed, “the tendency for the subduction interface to develop spontaneous thickness variation as the models evolve” (line 578) is already present in the models (see e.g. Fig. 2 and S4) hence it is not a “discrete isosurface” (line 58).
- some informations are missing in the section on model set-up : sediment layer and basaltic crust densities (same as lithospheric mantle?), thickness of lithospheric layer below plate core (refer to “slab” on line 146) – 45 km ?
- change the sentence “All other parameters are kept the same among simulations” (line 151) since the weak crust thickness varies from 10 km (when hsed=5 km) to 5 km (hsed=10 km).
- the sentence “We also consider constant sediment fluxes at the trench” (line 152) is not clear. The authors maybe refer to constant sediment flux through time, but this flux differs between simulations (since sediment thickness is varied, which changes convergence velocity).
- the sentence “a no-sediment case is represented by high-sediment viscosity (i.e., higher proportion of crust at the interface)” (line 168) is confusing. I suggest instead: “the highest sediment viscosity (eta_0) is equal to the weak crust viscosity, hence analogue to a no-sediment case”.
- the sentence “the trench is retreating, specific to ocean-ocean subduction” (line 183) is uncorrect: some intraoceanic subductions exhibit trench advance in nature (e.g. Izu-Bonin).
- please remove the 4-digit precision in Table 1 which makes the read more difficult. A round number suffices for Rc, alpha_wedge, W_wedge, and a 1-digit number is enough for u0, uT, h_trench, hmax and hmin. |