Review Reports - Escape Mechanism with Shallow Ramp and Décollements in Southwest Taiwan

Round 1

Reviewer 1 Report

The objective of this paper is studying the escape tectonics in southwest Taiwan, based on 3D numerical modelling implementation.

This is an interesting paper, which includes almost all major sections (Introduction, Numerical Solver and Models, Results, Discussion, Conclusions). Moreover, the “Numerical Solver and Models” and “Results” sections include additional sub-sections, providing a more detailed description. Regarding the mathematical part, related to the modelling procedure, it is valid, well=described and consistent with the extracted results. However, the following changes should be carried out, which will improve the paper:

Lines 81-87: The westward Anatolia microplate motion is a typical example of tectonic escape and therefore more details should be provided. Some significant papers, which analyze this tectonic escape are: 1. McKenzie, D., 1972. Active Tectonics of the Mediterranean Region. Geophys J Int 30, 109–185. https://doi.org/10.1111/j.1365-246X.1972.tb02351.x, 2. Dewey, J.F., Sengör, A.M.C., 1979. Aegean and surrounding regions. Complex multiplate and continuum tectonics in a convergent zone. Geol Soc Am Bull 90, 84–92. https://doi.org/10.1130/0016-7606(1979)90<84:AASRCM>2.0.CO;2. Regarding the GPS rotations of the same area, additional information can be found in the following papers: 1. Nyst, M., Thatcher, W., 2004. New constraints on the active tectonic deformation of the Aegean. Journal of Geophysical Research B: Solid Earth 109, 1–23. https://doi.org/10.1029/2003JB002830, 2. Lazos, I., Sboras, S., Chousianitis, K., Kondopoulou, D., Pikridas, C., Bitharis, S., Pavlides, S., 2022. Temporal evolution of crustal rotation in the Aegean region based on primary geodetically-derived results and palaeomagnetism. Acta Geodaetica et Geophysica 57, 317–334. https://doi.org/10.1007/s40328-022-00379-3. Please modify this paragraph and cite (optionally) the references mentioned above.

Lines 113-126: The paper objective is not clearly highlighted in the last paragraph of the “Introduction” section. Please, split this paragraph into two paragraphs (Lines 113-122 and Lines 122-126). In the last paragraph mention briefly the paper objectives (maybe numbering could be implemented). Please, apply.

Line 128: A “Methods and Materials” section should be included in the paper. Please, rename this section into “Methods and Materials – Numerical Solver and Models” or “2. Methods and Materials”, “2.1 Numerical Solver and Models”, “2.1.1 Numerical Solver” etc.

Line 187-199: The Figure caption should not be so long. Please, maintain the most significant information. The additional information could be merged with the main body of the manuscript.

Line 436: Please, rewrite the “Conclusions” section. In the current form it resembles an abstract. The major findings of your research should be highlighted and briefly described. Please, apply numbering.

Author Response

We agree with the reviewer. We’ve added the citations as [19,20,23,24] and added a few sentences to describe the tectonic escape in Anatolia. The description is as below:

For example, the tectonic escape of the Anatolian microplate is revealed by the modern geodetic measurements. The Anatolian microplate moves westward along the bounding strike-slip faults, the North Anatolian fault and the North Aegean Trough [23] due to the indentation of Arabia.

We’ve split this paragraph into two and modified the last paragraph to clarified our objective. The new last paragraph is as below:

This study aimed to discuss the escape tectonics in southwest Taiwan by 3D numerical modeling. We will (1) propose a thin-skinned model with shallow décollements, ramp and open boundary, which is inspired by geological observations, (2) present the simulated model evolution in time and space, and (3) compare the simulated ground motion with modern GPS observation to elaborate the regional deformation mechanism.

We’ve changed the title from “2. Numerical Solver and Models” to “2. Methods and Materials – Numerical Solver and Models.”

Line 187-199: The Figure caption should not be so long. Please, maintain the most significant information. The additional information could be merged with the main body of the manuscript.

We’ve moved most of the information from figure caption to the manuscript. The modified figure caption is:

Figure 3. Model setting. (a) The model is composed of two material layers and three lateral surfaces. (b) The depth of surface intersections in the initial mesh. (c) 3D perspective drawing of the two geometries in this study and the velocity boundary condition. (d) The velocity boundary condition at the eastern boundary. The velocity decreases downward from 50 mm/yr to 25 mm/yr at the lowest 1 km thickness.

We’ve rewritten the whole conclusion section as:

This research proposes a shallow escape mechanism for deformation in southwest Taiwan. We performed 3D numerical simulations of thin-skinned deformation with an open boundary at the continental slope, shallow ramp, and décollements. The open boundary is a free surface allowing materials to escape through which influences the stress field and changes the orientation of the main shear zone. Active slumping oc-curred around the open boundary in the early stage before the main shear zone devel-oped. The deformation further enhanced around the ramp due to the geometric effect. These early deformations weakened materials and changed the strength distribution. The main shear zone later developed across strength-inhomogeneous materials along the shallow ramp. The main shear zone broadens westward and disperses with the de-crease of material strength. Its strike rotates clockwise in the southern quarter. As a result, the main shear zone is curved, which makes the model velocity field fits GPS observation. We conclude that (1) the interaction of shallow structures, including the open boundary and shallow ramp, can lead to escape in thin-skin deformation; (2) an extremely low basal friction is necessary to reproduce the velocity field in southwest Taiwan; and (3) the angle between the convergence direction and the strike of the open boundary should not be large than 50°; otherwise the materials will move toward the open boundary without much change of direction.

Reviewer 2 Report

The manuscript is of a very good quality and proposes a new thin-skinned model for the tectonic escape in Southwest Taiwan, which brings insights to how the escape structure is influenced by the fold, ramp, and decollement geometry and the open boudanry based on the comparison with the GPS observation. The results obtained are original and important, therefore this manuscript deserves to be published in its current form.

On the form, I have a small problem with the discussion section. Perhaps it would have been more impressive to present previous study results on the escape structure below Taiwan based on modeling and then discuss the differences between the shallow escape structure in this study and previous results including the shallow and deep structure.

The scientific question is well established, then it becomes interesting to the community and researchers relative to this topic.

The proposed iconography is of very good quality and illustrates well the obtained results.

Author Response

To improve this part, we’ve added a new paragraph in “4.2. Extremely low Basal friction.” The paragraph is as below:

These major features are also reproduced by the previous 2D map-view model [33]. But in their model, the escaped block is between the deformation front (DF in Figure 1) and the Chaochou fault (① in Figure 2), not confined to the coastal region. In addition, the lateral bounding faults (DF and the Chaochou fault) have a low friction angle (15°) in the 2D model, compared with 30° in our model. The escape motion in our model is mainly facilitated by the extremely weak décollement so that lateral bounding faults do not have to as weak as that in 2D model.

Reviewer 3 Report

Review of the manuscript entitled "Escape mechanism with shallow ramp and décollements in southwest Taiwan“ Lee et al.

After reading the manuscript, I find this is an excellent and interesting study of the The escape structure in southwest Taiwan.This study provides 3D numerical modeling results to reveal the mechanical model for the escape in the southwest Taiwan region.

The data in this study are solid and trends in data and implications have been well-developed. The manuscript is well-written, well-organized, and the figures are appropriate, except for those few corrections that authors need to make before publication. I recommend this manuscript to be accepted by Geosciences after minor revision.

My only concern is to add a statistic analysis which is clearly missing in the text. This not only helps with defining the intervals but also will provide some uncertainties analysis.

Author Response

My only concern is to add a statistic analysis which is clearly missing in the text. This not only helps with defining the intervals but also will provide some uncertainties analysis.

We undertand the reviewer’s concern on the uncertainty. We’ve provided the standard deviation of model velocity in a new figure, Figure 8. The details about this analysis is described in the end of section 4.1. Comparison with GPS ground motion, as below:

The standard deviation of model velocity from 10 kyr, when the model reached a quasi-stable state, to 40 kyr is shown in Figure 8. The amplitude of the standard deviation is high in the slumping area and the shear zone. All standard deviations are significantly less than 1.5 mm/yr. We can conclude that the ground motion in the model is stable and can be safely compared with GPS observation.