the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Thrusts control the thermal maturity of accreted sediments
Utsav Mannu
David Fernández-Blanco
Ayumu Miyakawa
Taras Gerya
Masataka Kinoshita
Abstract. Thermal maturity assessments of hydrocarbon-generation potential and thermal history rarely consider how upper-plate structures developing during subduction influence the trajectories of accreted sediments. Our thermomechanical models of subduction support that thrusts evolving under variable sedimentation rates and décollement strengths fundamentally influence the trajectory, temperature, and thermal maturity of accreting sediments. This is notably true for the frontal thrust, which pervasively partitions sediments along a low and a high maturity path. Our findings imply that interpretations of the distribution of thermal maturity cannot be detached from accounts of the length and frequency of thrusts and their controlling factors. Taking these factors into consideration, our approach provides a robust uncertainty estimate in maximum exposure temperatures as a function of vitrinite reflectance and burial depth thereby reducing former inconsistencies between predicted and factual thermal maturity distributions in accretionary wedges.
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Utsav Mannu et al.
Status: open (until 22 Apr 2023)
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RC1: 'Comment on egusphere-2023-30', Jonas B. Ruh, 10 Feb 2023
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Review of the paper “Thrusts control the thermal maturity of accreted sediments” by Mannu and co-authors, submitted to Solid Earth.
In this study, the authors use a thermo-mechanical numerical model to investigate the thermal evolution, and in particular the thermal maturity, within forming accretionary prisms. The numerical model represents a mantle-scale subduction model and thus includes a (more) sophisticated thermal and isostasy model compared to higher resolution but dynamically simpler wedge models. Based on the thermal model and parameters specifically adjusted to fit borehole data from the Nankai through, the vitrinite reflection parameter %R_0 is computed based on three different existing models. The main conclusion of their work is that the evolution of %R_0 within accretionary wedges is strongly affected by thrusting, which is also observed in vitrinite reflection data from a borehole from the Nankai Trough.
The general idea of the paper is intriguing and allows to interpret the temporal and spatial evolution of a parameter, here thermal maturity through vitrinite reflectance, that in field measurements remains one-dimensional. This implies that the strength of the work is its applicability to natural systems, which comes a bit short. Below, I comment on several points that in my opinion need improvement for the paper to be accepted. I also attach the pdf of the manuscript with individual comments. The main points circle around the introduction of the numerical model and the comparison to natural data. Furthermore, there are many small errors and lack of clarity throughout the manuscript and writing has to be improved.
Based on the comments below and in the attached pdf, and my general impression, I recommend major revision before reconsidering the paper.
I hope my comments are constructive and helpful
Best wishes, Jonas
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Major comments:
1) Model setup. The paper consists to >90% of modelling results and therefore needs a proper introduction of how the model was set up and how the different routines are implemented. As the present paper is ultra-short, I see no reason why not to extend the model setup section by a proper introduction of the numerical but also the %R_0 model. For example move the model setup part of the Supp Mat to the main manuscript and show a general tectonic evolution of such a model including isotherms so that a reader that is not that familiar with geodynamic modelling can understand. Also better introduce the %R models and describe their differences. I personally would also have liked to see how %R is calculated to later better understand their differences (what is the temperature dependence etc.). Furthermore, there are some errors and ambiguities in the choices of parameters and the model description in the Supp Mat. First of all, décollement strength ranges from 2° to 22°, while internal wedge strength is unclear. Means, the wedge strength is defined by the faults, which after a strain of 1.5 have a friction angle of 15 or 20 (text and table differ). Furthermore, the tables itself are contradictory. Table 1 says décollement 0.03/0.08, which is nothing close to 22°, and sediment strength with a friction coefficient of 4.64, which is out of range. Table 2 gives friction angles that are not matching those parameters. Also, in the supplementary material, the equation for Mohr-Coulomb friction is wrong. I guess it should be Drucker-Prager (P in equation 9 is mean stress, not lithostatic stress as in the Mohr-Coulomb formulation), in this case missing a cos(phi) multiplied with cohesion. Otherwise one gets wrong geometric fault angles. I was also pretty lost with the sedimentation process. Although nicely introduced in the supplementary material, it remains enigmatic when only reading the manuscript.
2) Comparison to natural systems. The authors argue that the strength of the paper is its application to natural systems, but the paper only mentions one borehole to which it compares well. Since thermal parameters are implemented from that borehole that is not unexpected. Although the borehole data occupies a prominent position in this work, it is not really introduced. In my opinion, the paper would gain a lot of strength if it presented a proper section on comparison to previous work and natural examples on the topic. Also comparison to other numerical models that investigate thermal properties of shallow subduction zone dynamics is missing. For example Sepideh Pajang’s work in Solid Earth, as a counterpart to mantle-scale models (just an example).
3) Discussion. Large parts of the discussion are rephrasing the results and redundant. I think the discussion would benefit from a separation of subsections that focus on different topics, for example: Importance of thrusting on …., comparison to natural examples, comparison to previous work, and even implications for prospection or so, as it is mentioned to be of importance in the introduction.
Minor comments:
Besides the suggestions above, I commented directly into the attached pdf.
Utsav Mannu et al.
Video supplement
Movies for Paper Utsav Mannu, David Fernández-Blanco, Ayumu Miyakawa, Taras Gerya, and Masataka Kinoshita https://drive.google.com/drive/folders/1aPM2_s7bvyiZU1m0908qAvE79eJJ3GHb?usp=sharing
Utsav Mannu et al.
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