Peer review of: Oceanic crust recycling controlled by weakening at slab edges (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Report on the paper: “Lid recycling controlled by slab edge weakening” by J. Munch et al. 
 
The present paper employs 3D geodynamic numerical modelling to investigate the fundamental 
dynamics governing the evolution of slab roll-back intra-oceanic subduction zones, referring to 
natural examples such as the Caribbean, South Scotia and Gibraltar (Atlantic) Arcs. 
 
Numerical modelling is used to test the importance of two main parameters in the propagation of 
so-called STEP faults (Subduction-Transform Edge Propagator) that assist the subduction retreat 
migration, these are: a) the predominant rheological mechanism of the implied deformation (i.e. 
existence vs. absence of strain-induced weakening - SIW); and b) the existence vs. absence of 
bounding passive continental margins (as to see if the geometry of these margins could drive the 
lateral propagation of the subduction zone). 
 
The propagation direction of STEP faults (either complying with a convergent or a divergent 
geometry) is critical, since it would ultimately determine whether this type of subduction zone is 
prone to progressively narrow, leading to the (“necking”) break-off of the subducting slab, or 
alternatively to the widening “expand” of this subduction along the circum-Atlantic passive 
margins. 
 
The 4 main conclusions provided by the paper are: 
 
1) The main factor governing the geometry of propagation of STEP faults is strain induced 
weakening (SIW): this weakening renders STEP fault convergence, whether its absence leads to 
the widening, lateral propagation, of the subduction zone. 
 
2) The existence of passive continental margins laterally bounding the initial subduction-arc bears 
no relevant influence on the propagation geometry of STEP faults and ensuing subduction-arc 
evolution. 
 
3) Present-day subduction arc geometry (e.g. Atlantic South Scotia and Caribbean arcs) is 
preferentially compatible with fault-controlled fluid-driven chemical weakening (promoting SIW), 
along more discrete (localized) STEP domains, as would be expected to occur in a colder stronger 
lithospheric lid (such as the presently existent one). 
 
4) Given the observed compatibility with Present-day natural examples, Atlantic subduction arcs 
are not expected to evolve in a way as to propagate this subduction along its margins (and thus to 
invade or “infect” the Atlantic), but rather to undergo future (“necking”) slab break-off. 
Conversely, a relatively hotter and dryer lithospheric lid (preventing or strongly hampering SIW) in 
the Archean could have favoured a scenario of widening lateral propagation subduction evolution 
in that geological past. 
 
 
I find this paper extremely important. It is a new, well-argued, contribution to understand the 
dynamics (the main rheological mechanism) assisting STEP fault propagation, and thus, ultimately, 
the tectonic evolution of isolated subduction arcs within major oceanic basins. 
 
This bears the huge implication of trying to know if these arcs could correspond to the initial locus 
of (lateral) propagation of subduction zones (along the pristine passive margins of the 
encompassing oceanic basin), potentially leading to the closure of a main ocean. As such, it has 
implications for the understanding of the evolution of Wilson and Supercontinent Cycles, as well as 
for the knowledge regarding the workings of Plate Tectonics through geological time (e.g. 
regarding the issue of trying to know whether the presently observed plate configuration was set 
at a relatively early stage of Earth evolution, and has since then acquired a steady-state 
configuration, or if, on the contrary, such a configuration is prone to continuous change and 
evolution). 
 
The (3D numerical modelling) methodology is supported by well-designed models adequately 
conceived to address the two main investigated parameters. Noticeably, these models are fully 
dynamical, i.e. the forces driving the resulting geometry/kinematics are not externally imposed to 
the model, but rather are exclusively the result of the buoyancy contrast and viscous resistance 
between the different prescribed model domains/units. 
 
The paper is well written, well-illustrated, and cites the most relevant previous contributions. 
 
As such, in my opinion, the present work will be of interest for a broad audience of researchers 
and (even) for Earth-Science educated non-specialists. It absolutely merits publication (as soon as 
possible) in Nature Communications. 
 
 
I have only two main points that I think the authors should consider: 
 
 
1. I do not fully agree with the assertion that the study subduction arcs cannot evolve as to 
propagate laterally, sub-parallel to the passive margins of the confining main ocean, ultimately 
triggering the future closure of the latter. In fact, the investigated parameters are not the only 
ones governing the tectonic evolution of these arcs. Noticeably, this type of arcs in the Atlantic are 
migrating (retreating) towards the inner domains of the main ocean, i.e. towards the middle 
oceanic ridge. As such, the outer convex front of the arcuate subduction zone is progressively 
encountering younger, hotter and more buoyant oceanic lithosphere, which is less prone to be 
subducted. Contrastingly, at the extremities of the arc, the subducting oceanic lithosphere is 
further away from the oceanic ridge, and thus is older (i.e. relatively colder and denser) and hence 
more prone to undergo subduction. Note that these arc extremities correspond to the domains 
where STEP faults have to propagate to assist arc retreat. Nobody knows what would happen, or 
what will happen, when (e.g.) the Scotia Arc will get closer to the mid oceanic ridge, but it is 
conceivable that as increasingly younger oceanic lithosphere (at its convex front) becomes harder 
to subduct the arc would widen (open?) through lateral propagation of the subduction zone. If this 
process of formation of widen intra-oceanic subduction is possible, then it could also be nucleated 
at the other two Atlantic oceanic arcs (Caribbean and Gibraltar), potentially creating a network of 
extended subduction zones, and allowing to speculate in favor of the future closure of the Atlantic. 
In the present models, outside the younger window of overriding lithosphere, all (potentially) 
subducting lithosphere has the same original age (40 My) all around/near the arc trench. This 
prevents investigating the dynamic impact of having younger subducting lithosphere at the arc 
front, and older at its extremities, in the evolving arc retreat migration (including possible effects 
on the propagation of the STEP faults that assist such retreat). I agree that such an approach 
would be out of the scope of the present work (it would correspond to a new paper by itself alone), 
but I think this matter would merit some brief discussion while addressing the limitations of the 
present models. 
 
2. In their models the authors consider a maximum basal depth of only ~390 km (to which an 
open bottom boundary condition is prescribed), thus not including the upper-lower mantle 
transition at 660 km. This transition has been known to influence the dynamics of the subducting 
slab, specifically regarding the evolving shift between slab roll forward and slab roll-back, and the 
achievement of a trench-retreat steady-state (e.g. Stegman et al., 2010). As such, the interaction 
between the slab and the 660 km transition could also be meaningful for the dynamics of surface 
arc-propagation, namely since the bending of the retreating slab due to mantle toroidal flow would 
interfere/interact with the bending caused by the arrival of the subducting slab to the upper-lower 
mantle transition. This limitation of the models (i.e. absence of 660 km upper-lower mantle 
transition) would perhaps also merit some brief discussion. 
 
Some other minor points regarding the Methods section (besides very few other directly annotated 
throughout the original ms): 
 
a) Please clearly state what is the original age of the prescribed window of younger overriding 
lithosphere. 
 
b) Clearly state that the model is fully dynamic (only driven by internal buoyancy contrast and 
viscous resistance forces), not subjected to any external forces. 
 
c) Model original state: please clarify if is there an initial portion of the slab prescribed to be 
already subducted in the experimental initial state. If so, please specify the angle and maximum 
depth of such initial state subducting slab. 
 
Filipe M. Rosas 
IDL, University of Lisbon 
 
Refences cited: 
 
Stegman, D. R., Schellart, W. P. & Freeman, J. A., 2010. Competing influences of plate width and 
far-field boundary conditions on trench migration and morphology of subducted slabs in the upper 
mantle. Tectonophysics, 483 (1-2): 46-57. doi.org/10.1016/j.tecto.2009.08.026 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
The presented work is original and innovative: it explores the role of strain weakening in the 
development of slab tear propagation near STEP-faults. The numerical models are of the highest 
scientific quality. The paper is very well-written. The ideas in the paper regarding the evolution of 
subduction during Earth history are thought-provoking but, as detailed further below, insufficiently 
substantiated as yet because the connection between the models and particularly present-day 
STEP-bounded subduction systems is poorly developed. 
 
The strain weakening law is a critical ingredient in the numerical experiments. It refers to an 
aspect of rock material behavior that is commonly used in numerical modeling studies to induce 
localization. It is not directly constrained by independent observations, e.g., from rock mechanical 
experiments. Given the uncertainty surrounding the strain weakening parameters, the authors 
understandably vary them. They find substantial and interesting variations in the predicted tearing 
(diverging, straight, or converging). 
 
In line 46-7 it is written that “The aim 46 of this study is to investigate which of these scenarios is 
typical on present-day Earth”. What is conspicuously missing in the study is a comparison to 
present-day observations, which would allow the authors to conclude whether strong or weak 
strain weakening dominates at a dozen or so STEP-faults. A onset towards a comparison is in 
sentence 80-82: “models without SIW show the development of a wide straight arc with a deep 
(>9 km) trench that looks unlike the Scotia subduction zone or other modern narrow retreating 
subduction zones examples (cf. Figs. 1 and 2.f.)”. The correspondence between Figs.1 and 2f is 
not so apparent to the reader as suggested here. As the emphasis of the paper is on Girbraltar, 
Scotia and Caribbean STEPs, recent geological indicators of STEP convergence need to be 
discussed here and a semi-quantitative comparison to the model predictions needs to be made. 
Wide subduction systems like Tonga and New Hebrides do not seem to show STEP-convergence, 
making me wonder whether the model experiments are valid only for rather narrow trenches. In 
terms of the paper structure, benchmarking these critical weakening parameters is a logical step 
before backstrapolating to historical settings, and before speculating about the emergence of plate 
tectonics. 
 
Spakman and Wortel defined the “STEP Fault” as the strike-slip boundary between the two surface 
plates, one of which is the overriding plate. This definition appears to be widely followed in the 
literature. The “free subduction” numerical models in the paper do not have an overriding plate so 
there are no STEP-faults in these models, but tears. 
 
Could the prediction that the present Atlantic subduction zones will cease to exist be affected by 
an overriding plate? Please discuss particularly the potential sensitivity of mode III fracturing to 
this; my first thought would be that out-of-plane also will require deformation of STEP-fault 
adjacent parts of the overriding plate, and that mechanical resistance to such deformation may 
affect the tearing in this coupled system. 
 
Opening sentences (L.33-37). “The puzzling emergence of plate tectonics and the maintenance of 
a mosaic of coherent 33 plates remains one of the key contemporary research questions”; this is 
absolutely true, but the contribution of the current research to answering this question is very 
limited. 
 
Figure 1. The southern end of the Scotia STEP may actually not be tearing because further to the 
east there already is a plate boundary discontinuity. I looked it up in Govers and Wortel (2005), 
and on page 510 they say: “Beyond the STEP further to the east, the South America and 
Antarctica plates are direct neighbors along an east–west oriented transform plate boundary 
(South America–Antarctica (SA- A) ridge). This has the consequence that STEP fault lengthening 
here occurs along a pre-existing plate boundary.” 
 
In Line 60, add “free-subduction” as an adjective to “models”, this clarifies an important aspect of 
the approach. 
 
Line 118, specify what “critically” means. 
 
L.118. This confirms the conclusions of Nijholt, N., & Govers, R. (2015). The role of passive 

margins on the evolution of Subduction‐Transform Edge Propagators (STEPs). Journal of 

Geophysical Research, 120(10), 7203–7230. It would be fair to credit these authors here. 
 
L. 163: State explicitly and motivate that rock elasticity is neglected. Such rigid-visco-plastic 
rheology differs from actual bulk rock properties, but is it a relevant difference? 
 
Sentence L.213-4: “We use a grid resolution of 2 and 3 km in the vertical and horizontal 
directions, respectively.” There is no motivation of these choices and the dependence of the results 
on the grid size and time step which would be appropriate in this methods section. 
 
L.239 The authors assume the same water content for oceans and continents. There is a 
substantial literature starting from Hirth and Kohlsted suggesting that oceanic lithosphere initially 
is much drier.