Impact of upper mantle convection on lithosphere hyperextension and subsequent horizontally forced subduction initiation

Candioti, Lorenzo G.; Schmalholz, Stefan M.; Duretz, Thibault

doi:https://doi.org/10.5194/se-11-2327-2020

Articles | Volume 11, issue 6

https://doi.org/10.5194/se-11-2327-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/se-11-2327-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 11, issue 6

Research article

|

07 Dec 2020

Research article |

| 07 Dec 2020

Impact of upper mantle convection on lithosphere hyperextension and subsequent horizontally forced subduction initiation

Lorenzo G. Candioti, Stefan M. Schmalholz, and Thibault Duretz

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Final revised paper (published on 07 Dec 2020)
Preprint (discussion started on 25 May 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Referee comment for manuscript by Candioti et al.', Anonymous Referee #1, 26 Jun 2020
- AC1: 'The authors reply to the referee 1', Lorenzo Giuseppe Candioti, 04 Aug 2020
RC2: 'Comments on the manuscript', Anonymous Referee #2, 07 Jul 2020
- AC2: 'The authors reply to the referee 2', Lorenzo Giuseppe Candioti, 04 Aug 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Lorenzo Giuseppe Candioti on behalf of the Authors (12 Aug 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Aug 2020) by Susanne Buiter

RR by Zhong-Hai Li (16 Sep 2020)

Suggestions for revision or reasons for rejection

In the manuscript by Candioti et al., a series of 2D thermo-mechanical models are used to study the successive processes from continental break-up and oceanic lithospheric cooling to the following subduction initiation under compression. In addition, the mantle convection is included in the model, which is shown to play important roles in regulating the whole model evolution, especially may facilitate the formation of asymmetric, one-sided subduction initiation. The study is interesting and the clearance of the manuscript is greatly improved by considering the comments/suggestions in the first round of review. Thus, I believe it is worth for the final publication. Since it is the first time for me to read the manuscript, I do have some additional comments for further improvement of the paper as shown below.

Major points:

(1) Sections 3.3-3.5, comparisons of M1-M5: The one-sided versus two-sided subduction initiation (SI) is an interesting point of the study. The authors propose that the suction force induced by asymmetric down-welling controls the one-sided SI in M1. However, on the one hand, how large is the suction force in M1 comparing to those in other models M2-M5? On the other hand, it is still not clear about the dominance of this force comparing to other factors, e.g. the asymmetric passive margin, for the strain localization. In Figure 8, it clearly demonstrates that the strain localizations in M1 and M4 are both focusing on the single margin. I am curious why the single-sided SI is only developed in M1, but not in M4 (Figure 9). It may help better understanding the processes by checking the continuous movie of the model evolution. Please add a paragraph for discussion.

(2) Section 3.6 and Appendix D, plate driving forces: The calculation of plate driving force is not very clear. Why is the Tau_II(x) integrated vertically over the whole model domain, ie. from the model bottom to the top surface (Equation D3), but argued in the main text that “the vertically-integrated second invariant of the deviatoric stress tensor Tau_II is a measure for the strength of the lithosphere”? I understand the deviatoric stress should be mainly localized in the lithosphere. Then why does it need to integrate over the whole model domain down to 660km? Could you compare the exact values?

(3) An important finding of this study is the decreased pushing force (ie. 15 TN/m) required for SI at passive margins, comparing to the previous works with less heterogeneity (e.g. 37 TN/m in Kiss et al., 2020). However, the force still seems to be large. As shown in the current models, the mantle convection induced force could be about 2 TN/m. With the possible ridge push force of another ~3 TN/m (Turcotte & Schubert, 1982), it is still not enough. What kind of additional forces can be used to explain the prescribed boundary convergence? How about the suction force?

Minor points:

- Title: The phrase “convergence-induced subduction” may be modified to “convergence-induced subduction initiation”, because only the subduction initiation process is studied.
- Abstract: The structure of the abstract is rather loose. For example, from the 3rd line to the 8th line, many sentences are used to describe the details of the model setups, which could be shortened. In addition, the organization of the main findings is also not very clear, which may be more logically organized as: 1) Our model generates a 120 Myrs long geodynamic cycle of subsequent extension (30 Myrs), cooling (70 Myrs) and convergence (20 Myrs) coupled to upper mantle convection in a single and continuous simulation. 2) The model results indicate that “xxx”, where to show the impact of upper mantle convection on lithosphere hyper-extension, as highlighted in the title. 3) The subsequent, compression-induced subduction initiation should be horizontally forced, rather than vertically, pure gravity forced. 4) The single-sided SI versus double sided SI, as well as the controlling factors, i.e. the mantle convection and others. 5) The forces required for SI and related aspects. 6) Geological implications.
- Lines 30-35, “The 660-km phase transition can therefore represent a natural impermeable boundary, that”: I do not think it is impermeable, since it is just a certain resistance due to the viscosity jump and a negative Clapeyron slope.
- Lines 73-75, rephrase.
- Line 124, “as mentioned above”: which is it and where is it? It is too far away from this point.
- Line 135, “In these models”: which models are they? The models you referred to in the last sentence, or the models you have conducted.
- Line 145, “The top surface is stress-free”: are you using a free surface? Is the sticky air still used, since there is a 20-km domain above the crustal surface.
- Section 3.1, Model-M1: the different margin widths of 200 and 150 km. I am wondering 1) is the model setup purely symmetric, and 2) the effect of the contrasting margin width on the single sided SI.
- Section 3.2, Model-M6: Actually, I do not know the purpose of this model with rather weak upper mantle and the resulting unstable lithosphere, which is definitely contrasting to the processes studied in this paper, ie. extension-cooling-SI. In the discussion of the paper, I finally find that this model is compared a bit to the Archean Earth, which is also not related to the focus of this study.
- Section 3.6, FD is defined as ‘per unit length’: per unit length along-strike?
- Line 298, “At ca. 105 Myrs, values for FD increase again until the end of the simulation.” Why? Since the subduction has already formed, which should ‘release’ the force.
- Section 4.1: I agree that it is very difficult for the passive margin to collapse spontaneously. However, a weak zone is generally considered for such kind of spontaneous SI model (e.g., Stern and Gerya, 2018, Tectonophysics), which may be briefly noted.
- Line 329, “In our models, subduction is initiated self-consistently”: I do not know whether it can be regarded as ‘self-consistently’, since the model is pushed by prescribed boundary velocities.
- Line 385, the suction force is very large. Firstly, the approximation of suction force is kind of dependent on the size of the domain for integration (Equation D4). Secondly, how much of the gravity anomaly induced force (defined as suction force in this ms) acting the passive margin, ie. to facilitate SI. It may be difficult to quantify, but a short discussion may be necessary.
- Line 505, the divergent subduction of Adriatic plate: Did the SI on both sides start at the same/similar time? If they are not, then it is not quite applicable to the current model.

Zhong-Hai Li
University of Chinese Academy of Sciences

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ED: Publish subject to minor revisions (review by editor) (16 Sep 2020) by Susanne Buiter

AR by Lorenzo Giuseppe Candioti on behalf of the Authors (05 Oct 2020) Author's response Manuscript

ED: Publish as is (07 Oct 2020) by Susanne Buiter

ED: Publish as is (07 Oct 2020) by Susanne Buiter (Executive editor)

AR by Lorenzo Giuseppe Candioti on behalf of the Authors (14 Oct 2020) Manuscript

Short summary

With computer simulations, we study the interplay between thermo-mechanical processes in the lithosphere and the underlying upper mantle during a long-term (> 100 Myr) tectonic cycle of extension–cooling–convergence. The intensity of mantle convection is important for (i) subduction initiation, (ii) the development of single- or double-slab subduction zones, and (iii) the forces necessary to initiate subduction. Our models are applicable to the opening and closure of the western Alpine Tethys.