Active Strike-Slip Faulting and Systematic Deflection of Drainage Systems along the Altyn Tagh Fault, Northern Tibetan Plateau

Round 1

Reviewer 1 Report

The authors, through detailed analysis of DEM images, mapped left-lateral offsets of streams and rivers along the Altyn Tagh fault (ATF), northern margin of the Tibetan Plateau. Because they found a maximum offset of only 72 km, they argue that tectonic extrusion following the India-Asia collision is limited.

While the figures are really nice, and the study is potentially interesting, I find the problem rather basic and the conclusions drawn from the study not appropriate. The authors seem to completely ignore the fact that geological offsets exist, and that their study only focuses on late Quaternary offsets, which of course, are much more recent than those from granitic pluton etc, hence much smaller. Therefore, their conclusion that the maximum offset along the ATF is 72 km and not 500 km is unfounded. They compare things that are not at the same timescale. This should at least be acknowledged and discussed.

In addition, the authors claim that the minimum gully offset along the entire ATF is 7 m and this is co-seismic. This is incorrect as only M>8 earthquakes can produce such large co-seismic offsets. The fact that they used 90 m resolution DEM is probably responsible for the fact that they do not report smaller offsets. For sure, many smaller offsets exist. You can see such smaller offsets in the numerous tectonic-geomorphology studies on the ATF, that are not cited in this paper, but should. This fault is one of the best documented in Tibet.

None of the offsets reported in this paper have uncertainties. Please add in Figures 2-7 and Table S1 as this is a scientific paper after all.

My biggest concern is that the references are not appropriate. The authors heavily rely on old publications, which is not a problem per se, but seem to ignore the more recent ones. The ATF has been studied by many, on the various topics mentioned here, from geologic rates, late Quaternary rates and geodetic rates, using GPS and InSAR. A good way to acknowledge these would be to show the location of all studies in their Figure 1 for example, in addition to showing the reported offsets and rates. That way, we can see that the minimum and maximum offsets (at all timescales) are not the range proposed here (7 m – 7 km) but maybe something lie 3 to 500 km. Note that depending on which segment of the ATF is considered, these may vary. While the authors do not rely on field work at all but just on DEM and Google Earth images, they can find smaller offsets in the relevant literature.

Lastly, the English should greatly be improved, especially in the introduction section. The current language level is not acceptable for a journal such as Remote Sensing.

Therefore, I recommend major revisions and that all these comments be properly addressed.

Regards,

Abstract

I would not call the ATF, “one of the largest active strike-slip faults in the world with a length of ~2000 km” as you write in the geological setting, a “regional fault” but rather a “large-scale fault”.

Introduction

First, I highly recommend to cite the numerous studies of offsets along large strike-slip faults from the Tibetan Plateau rather than studies from California, New-Zealand, Japan, Iran etc.

Second, I find the introduction too general and vague. Please be more specific with concrete examples and broaden the impact of your study, which as it reads, rather limited.

L74-76:

-I doubt that REF #12 on the San Andreas fault discusses the 500-750 km offset on the ATF.

-There are many more studies than #21 and #22 you should cite here.

-Also, please show this large-scale offset on a map.

-There might be a confusion here, your study focuses on river and gully offsets while long-term studies focus on geological offsets. Of course, you cannot obtain offsets of 100s km with late Quaternary geomorphic features. This needs to be better addressed.

Fig. 1:

-I recommend you show all earthquakes of M>5 as colored dots, not just those for which you have a focal mechanism. That would also include the two large ones in 1924 in the western ATF.

-in the caption, define RRF, YTR, YLR, RR.

-no LMS in A.

-A is too small to read the labels. Enlarge.

L96: there are many more studies to cite here.

L115: “a minimum slip rate of ~5 mm/yr was argued by the geological mapping and geomorphic studies carried out by the “Altyn Tagh Active Fault” research group [33].”

This was done 40 years ago and many groups have studied the ATF since and more accurate rates have been determined. This citation is thus obsolete. Remove.

L92-124: I highly suggest you plot or summarize all these rates on a map (Figure 1?) or in a table at their correct location as it is heavy to read, especially for readers unfamiliar with the region.

Methods

L127: write the proper REF for “Russel” after its name.

L140: Hayward fault

L163: “The minimum value of ~7 m recorded by gully AT076 probably indicated the most recent seismic event.”

As you might know, no large earthquake on the ATF has been reported in the last several hundred years. However, in order to produce a co-seismic offset of 7 m, only a M>8 earthquake could do that. Also, there are many offsets of 2-3 m. However, using 90 m resolution DEM may not allow you to see them. You should at least acknowledge this limitation of your study.

L167-174: “Our interpretation confirmed that rivers with a large order of magnitude tend to accumulate larger offset amount.”

Of course, the larger the river the older it is, so the larger the offset! This is not new.

Figure 2:

-add coordinates

-indicate that the offsets are in meters

Figure 3:

-add coordinates

-Add fault trace in B

Figure 9 and 11:

-What is the difference between the left and right panels. Please explain.

-Is B a close-up of A?

Figure 10 is missing

Discussion

L296: show the deflected ranges, terraces and fans in the figures mentioned.

L337-344: Why such a difference? the Haiyuan fault is located in similar climatic conditions as the ATF. Also, the ATF is very long so it would be wise to differentiate, if any, the erosion rates in the different segments, which might be influenced by different climatic conditions. This needs to be discussed.

-How did Craddock et al. calculate headward erosion rates?

You need to explain this in detail if you want to compare their data with your data.

L372: again, cite more studies.

L399: how much larger? Give an estimate.

Author Response

   We are grateful to #1 reviewer for his/her constructive and critical comments and suggestions that greatly helped us to improve the manuscript. We have revised the manuscript by taking into account the reviewer’s comments and suggestions by point to point reply. The details are shown in the attached file named "Reply to 1#Reviewer.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript introduces the results of a morphotectonic study of the drainage network geometry along the Altyn Tagh Fault, northern Tibetan Plateau aimed at the estimation of long-term offset recorded by deflected channels. Based on the photo-aerial interpretation of remote sensing images and DEMs, the Authors infer a systematic sinistral deflection of bedrock and alluvial channels along the trace of the left-lateral strike-slip Altyn Tagh fault zone, which has been used as morphotectonic marker of long-term analysis of the fault magnitude. The paper introduces a large amount of morphotectonic data but I have some concerns about the rationale and approach of the paper. In my opinion, the research design and the basic assumptions of the proposed approach should be better demonstrated. The authors assume that left-lateral strike-slip faulting causes a systematic deflection of channels but this simple response of the drainage network is not so obvious (see for example Walker & Allen, 2012). Although several works have demonstrated that strike-slip fault can deflect a stream by dragging a reach on one side of the fault, plano-altimetric distribution of offset channels can not show simple linear geometry due to complex fluvial processes, irregularities of the fault trace, perturbations induced by climate, sediment budget, local geomorphological processes and angle between channel and fault trace (see for example Ouchi, 2005, Geomorphology). I think that the basic assumption and its robustness should be better emphasized in order to clarify if the approach of the paper can be suitable for the purpose of the work. Moreover, I suggest to systematically integrate the analysis of planform geometry of the drainage network with other useful and consolidated morphotectonic markers such as longitudinal profile analysis, knickpoint distribution, fluvial terraces, and alluvial fans. Such an approach can furnish additional and crucial information about the robustness of the results of the morphotectonic analysis.

Author Response

   We are grateful to #2 reviewer for his/her constructive and critical comments and suggestions that greatly helped us to improve the manuscript. The rationale and approach of this study are interpreted below, we hope this interpretation can satisfy you. The details are shown in the attached file named" Reply to 2# Reviewer", please see the details.

Author Response File: Author Response.docx

Reviewer 3 Report

In this manuscript, the authors study the relationship between strike-slip faulting and offset cumulation of drainage systems. Using high-resolution remote sensing images and three-dimensional steroscopic inspected technology, the seismic recurrence characteristics of active faults were revealed by measurements and analysis of displacement amount of rivers or gullies.

The manuscript fits well into the scientific scope of the journal Remote Sensing and presents new, relevant and interesting data and findings.

The manuscript is altogether well written and well presented.

I have only a few suggestions for (minor) changes and additions:

• In the end of section 1. Introduction: Please formulate clear research questions and aims here (rather than summarizing what you did)
• In the Conclusions section: I recommend to include a few sentences on wider and internationally relevant implications of your interesting scientific findings. This will further increase the impact of your paper. 

Author Response

    We are grateful to #3 reviewer for his/her constructive and critical comments and suggestions that greatly helped us to improve the manuscript. We have revised the manuscript by taking into account the reviewer’s comments and suggestions by point to point reply. The details are shown in the attached file name "Reply to #3 Reviewer".

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear authors, 

I have now read the revised version of your manuscript. I am happy with most of the changes even though more could have been done. While the language is still not good, it is readable and understandable.

Author Response

Dear reviewer,

please see the attached file named "Round 2 reply to 1#Reviewer"

Reviewer 2 Report

The manuscript has been improved according to the suggestions and advices of the first revision but I have still the same concern about the rationale of the paper. Valuable works (for example Walker and Russel, 2012 and Ouchi, 2015) that questione the simple response of a systematic deflection of channels in response to the activity of strike-slip fault have not been introduced and a fuller description about the possible complex interplay between geomorphological and tectonic control factors such age and evolution of the channels, intermittent fault activity and role of local topography or sediment flux is lack. The basic assumption of the systematic channel deflection in response to fault activity should be demonstrated by introducing examples of field cases where a channel restoration is evident and confirmed by independent morphotectonic markers such as offset of other geomorphic elements or anomalies of longitudinal river profiles. Morphotectonic analysis should include the integration of different morphotectonic indices and markers in order to reduce the uncertain due to feedback between tectonic signal and other complex external factors. For example, analysis of spatial distribution of knickpoints/knickzones in river profiles could be useful to detect phenomena of fluvial captures or perturbations induced by tectonic forcing. Finally, I suggest to fully discuss the diagrams of Figures 9, 10 and 11: although the correlation coefficient of two of the three plots is high, I think that the plots show poor linear relationships for the higher values of the upstream length.

Author Response

Dear reviewer,

please see the attached file named "Round 2 reply to 2#Reviewer"

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

The manuscript has been improved and it can be suitable for publication