Evidence of Dextral Strike-Slip Movement of the Alakol Lake Fault in the Western Junggar Based on Remote Sensing

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The NW-SE trending dextral strike-slip faults on the north side of the Tian Shan play an important role in accommodating the crustal shortening. The Alakol Lake Fault, that stretches for roughly 150 km from Lake Ebinur to Lake Alakol, displays obvious right-lateral strike-slip active faulting. However, the formation process of this fault and its connection with the Dzhungarian fault have not been well studied, which hampers our understanding of how the Tian Shan moved and evolved. The manuscript of “Evidence of dextral strike-slip movement of the Alakol Lake fault in the western Junggar based on remote sensing” identified the dextral strike-slip characteristics of the Alakol Lake fault using high-precision aerial photographs, satellite stereo-imagery, and field observations. Then, the authors described the detailed fault traces and tectonic features such as alluvial fan surfaces, gullies, offset stream channels, and fault scarps based on the obtained data. Finally, they discussed the role of the Alakol Lake fault in the tectonic pattern formed in the north of Tian Shan. The manuscript is very well-written, and clear in its aims, methods and results. I therefore suggest that it can be published after several modifications and clarifications. Detailed comments and suggestions see below.

 

1、In Figure 2, the authors marked the national boundary using white dashed lines, I strongly urge that the authors delete these lines.

2、In Figure 5, the field photos of 5c and 5d show the fault trace and the pull-apart basin. However , the pull-apart basin can not been identified on the photos, I suggest the authors replace a photo taken by drones.

3、In the manuscript, the authors obtained the strike-slip offsets by measuring the displaced stream channels and/or gullies, that usually record a minimum value of offsets after the landform abandonment. I therefore the authors remove the contents about the ratio of horizontal displacement to vertical displacement. Based on the displaced landforms, the readers can easily determine dextral strike-slip-dominated movement characteristic.

4、For site 4, the authors described the fault displace the T2 riser of 20±3 m. I suggest that the authors enlarge the local displaced landforms and provide detailed interpretations.

5、At site 5, the authors measured a 13 ± 1 m dextral slip offset of the T2 riser, however, the dextral slip offset of the channel reaches to 20 ± 4 m. I can not understand these strike-slip offsets. In addition, as described by the author in the manuscript, two fault branches at this site, the authors only measured one fault branch of strike-slip offset.

6、The authors propose a deformation pattern of crustal shortening adjustment model. However, GPS data give a deformation image of counterclockwise rotation of the Junggar and Kazakh blocks. How does the author consider this issue? 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This paper is a great application combining remotely sensed elevation data and field-collected OSL data to constrain fault slip rates in a new area.  The methods are sound, and the kinematic interpretations should be of broad interest to workers studying the India-Asia collision.  The conclusions are well supported by the data.  The paper is well written, and has clear figures.  I have only minor suggestions for improvement or clarification, including a couple places that appear to be missing citations, which can be found annotated onto the attached pdf.  Overall it is a strong contribution and I look forward to seeing it published.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English language is mostly good (see a few minor suggestions on pdf), although I did detect a higher rate of awkward phrasings starting about halfway through the discussion, so you might want to review the language near the end of the paper a bit more carefully.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Based on detailed indoor high-resolution images interpretation, the field  observation and results of field dating samples, the faulting characteristic and slip rate of the Alakol Lake fault has been obtained. These conclusions, providing the reliable geologic slip rates and pattern of the connecting fault,  are crucial for regional strain distribution model and its deformation model. I have already marked out my commment in the attached PDF file. The author could revise the manuscript following the remarks. 

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

The authors present important new geomorphic observations and measurements on the activity and kinematics of the Alakol fault, a significant strike-slip structure embedded within a major right-lateral shear zone that puzzlingly transects the Tien Shan. The authors present topographic analysis based on photogrammetric datasets they collected and derived to measure fault offsets, and they present some limited luminescence dating of burial ages of surface deposits to gain some crude constraints on rate and timing of activity. This information is valuable as a preliminary characterization of this structure, its role in the shear zone, and overall, as the authors suggest, the nature of the shear zone.

 

Several of the specific offsets (both horizontal and vertical) that the authors measure and report are only weakly justified and are somewhat suspect. At a general level, the authors must take more care in explaining what features they are using to measure offsets, how they are making those determinations, and how they derive uncertainties. In specific cases, noted in my line-by-line comments below, the authors present erroneous or indefensible “offsets” that either need much much more justification, or should be disregarded. Substantial ambiguity remains about the origin of deflection in channels they have observed and what exactly they are measuring to determine scarp heights. In particular, pay attention to erosion and inheritance in channel deflection magnitudes, and also strongly consider, justify, and explain how piercing lines are identified, correlated, and measured on either side of the fault.

 

This contribution is valuable, but the measured offsets and offset rates need to be presented with more detailed care and justification in order to be a defensible publication with rigorous conclusions on the slip rate.

 

Line-by-line comments:

15-16 - it would be useful to define what the Dzhungarian Gate is, as well as perhaps a longitude and latitude range or other geographic reference points to locate the study area

16 - de-capitalize satellite stereo-imagery

17 - further define “distribution” — of what, slip rates? trace locations, etc…

20 - “connecting structure” rather than “connecting point”

27 - is this conclusion based on the results presented here, specifically, of decreasing slip rates within this right-stepping right-lateral shear zone?

53 - this would be an appropriate place to explain how one might test between these two alternative models, or what the implications of each are for hazard or tectonics or geodynamics

67 - “seems to connect” does this mean there are direct structural links? or rather than it plays a role in accommodating shear between the two faults? It could be better to be more specific: the ALF appears continuous with the projection of the Chingiz Fault (or vice versa)

124-125 - it might be best to avoid the word “segment” which carries loaded meaning in the context of strike-slip faults. “Sections” is a more generic word, but this division across the international border is rather arbitrary, geologically…

126-127 - “green vegetation” is not a description of landforms, and may not be too relevant to the tectonic geomorphology you are studying. Could you describe the landforms themselves? Alluvial grasslands…

227 - 237 - check grammar: the sites did not use the various techniques, YOU used the various techniques at different sites

247 - elaborately would probably better be replaced by extensively

255 - reserved should be preserved

308 - please describe the basis for your assignment/assessment/interpretation of relative fan ages

309 - erosional-side offsets are notoriously unreliable, due to… erosion (see e.g., Cowgill 2007). The uncertainty of 8 m appears far too low, considering the curvature of the channel and the great distance 50 m? that the downstream piercing line must be projected to the fault trace. Furthermore, microtopography/small channels incised into the surface of fan 3 appear not to be offset this same amount.

310 - why is the younger fan 2 offset more than the older fan 3? This is not possible with uniform kinematics and/or indicates your assessments of offset are incorrectly inferred to be full offsets since the age(s) of abandonment.

314 - as measured, these offsets on fan 1 profila A-A’ would indicate a 10 cm downhill-side-up vertical offset: is this compatible with your expectations or interpretations at adjacent sites? the nature of these vertical scarps should be described or explained further, as it is not clear that they have significant meaning nor that they are reliably represented by the measurements you have made.

316 - as shown in figure 5b, the 5 m measurement comes from an upthrust wedge, apparently between the main fault and a backthrust or secondary strand. It clearly does not represent the net vertical offset across the fault which is better characterized by the portion of the profile between 0 and 100 m. This measurement does thus not reflect fault slip at a meaningful tectonic or seismogenic depth.

332-333 - explain the significance of scarp heights here. A-A’ and B-B’ are taken from different fan surfaces, and A is not associated with any lateral offset, apparently.

335 - replace word “filed” with “field”

356 - “separated into two strands”

352-368 - simplify your descriptions with consistent phrasing and basic facts; as written there appear some ambiguities about which fans reported measurements are from and how many fault strands are able to be constrained. This should be comprehensible without relying on the figure.

357 - “the stream channel was displaced” … The active channel is continuous across the fault, concealing its trace. The 20 m measurement indicated in the figure is based on a terrace riser below two non-correlative terrace surfaces, and is thus a challenging geomorphic marker (with two highly different and uncertain projection angles up- and down-stream). This needs to be described carefully and the logic of correlation and offset measurement explained in detail, in order to present a defensible right-lateral offset measurement.

359 - the 13 m displacement here is also rather suspect, as it appears to be defined by the top of a gully incised into the terrace riser. These offsets require much more rigorous defense.

360 - given such a high horizontal to vertical slip ratio found elsewhere, the topographic profile A-A’ is not meaningful as it does not connect correlative surfaces. T3 downstream is either translated away, eroded away, or potentially buried. If the latter, 16 m would be a minimum, but your horizontal-to-vertical slip ratios found elsewhere suggest that something with 16 m vertical displacement should also be translated > 100 m northward on the west side of the fault. This warrants careful explanation, and you should reconsider whether it is meaningful to present.

364 - “this” alluvial fan is imprecise. Why don’t you also name this adjacent fan? Expand the site description in this paragraph to indicate that you also surveyed the scarp on an adjacent fan

365 - “prove” is not the right scientific word to use. Similar scarp heights suggest similar ages of the fans.

386-387 - these offsets are more convincing, particularly because you have three that you expect to be compatible and they are. This warrants deeper explanation and more rigorous presentation of, for example, how the uncertainties were determined (note that they are only just BARELY compatible within uncertainty)

404-407 - this is another fair way to determine offsets and the uncertainties here seem more realistic, but the paragraph fails to describe specifically how you determine the fan offsets. Please describe how you map or determine fan axes and derive these uncertainties, both upstream and downstream (they will be different)

435-436 - total value of measured vertical displacement is age-dependent, not segment dependent unless you are comparing features of similar age. Rephrase this to indicate what you can really conclude; it is not meaningful to say vertical displacement is the same unless you indicate you are looking at features of correlative age. 

450 - which of the offsets reported earlier that you measured should be conveyed sooner, to indicate whether the measurements you’re making are useful for slip rates. Similarly, here in this section, indicate WHY you are dating each of these samples: for a slip rate? for age of activity? Tie this information together a little more informatively, not just presenting all the offsets then presenting all the dates separately.

468+ - the assumption that height-above-channel is the same for similar fans relies on the assumption that across the valley, the fans share a base level. Please include sentences justifying this.

 

Section 5.2 - vertical slip rates may not be super significant here, if the horizontal to vertical ratio varies by a factor of 2, on a predominantly strike-slip fault, which, as you have illustrated in your DEMs has abrupt local changes in scarp facing direction.

31 - This rate does not agree with the rate calculated at site 4. Why. These don’t represent the viable range, they represent an incompatibility that warrants explanation.

Comments on the Quality of English Language

The presentation is comprehensible; only in limited/few places did grammar or word choice errors hinder the comprehension of the scientific results. There is abundant overuse of articles ("the") which could/should be double-checked by a fluent English speaker. There are a few places, for example in the paragraph around line 227+, with incorrect grammar that should be remedied, as indicated in my line-by-line comments.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 4 Report

Comments and Suggestions for Authors

I thank the authors for their attention to comments previously raised and for the great improvements they have made to the presentation of this manuscript. 

While the presentation of information regarding offsets and uncertainties have been improved, problems remain with both how they are measured and how they are conveyed verbally. Specifically, while I appreciate that the authors have more explicitly named which measurements may not reflect the total amount of real displacement, due to intervening erosion of the offset markers, the language used to describe this needs to be improved further. For example: in revision line 346, "the alluvial fan was... displaced 34 +/-8 m at the erosional side, which is a smaller value than the real displacement..." is not the correct way to state this. The inherent problem is that we do not know the real displacement and thus cannot know that this is *not* the real displacement, we simply know that there may/could/probably have been erosion of this feature and thus the measured offset represents a minimum estimate of the real displacement. It would thus better be phrased, "The north edge of the ... fan exhibits an apparent dextral offset of ___, representing a minimum estimate of the displacement due to the possibility of erosion of the marker."

Throughout the manuscript these measurements are reported in the form "[such and such feature] was displaced by ___ m..." This is an incorrect and unscientific way to present this information, and edits should be made throughout to better convey that what you are presenting are measurements of apparent offset of markers, SOME of which you are interpreting to represent real displacements, or perhaps minimum displacements. Please edit further accordingly.

Furthermore, some of these measurements remain highly suspect, for example again at Site 3, the south edge of T2 is not convincingly offset along a fault strand. If the deflection you have annotated there represents displacement along a fault, an explanation must be given for why the landform curves continuously to connect up and downstream. There is no discrete offset of this marker as would be expected for the protected edge of a displaced riser. It thus should not be treated, as you have, as a reliable measure of "the real horizonal offset". Not simply every apparent right-lateral deflection can be interpreted directly as a meaningful measure of fault slip, and so more care must be taken to explain what is being measured, why it's being measured, and how it should be interpreted.

Relatedly, the diagram provided in the author's response document illustrating the offset measurement method shows a method that is unconventional and likely problematic. Offsets along a fault must be measured *at the fault* where the piercing line projects into it, not at some arbitrary distance away. Taking the average of projections from the opposite side of the fault far up and downstream from the fault may lead to widely variable results that do not reflect how much a feature has been displaced at the fault. The conventional and accepted way to perform these measurements is to present a range of possible projections of a feature to the fault, or a projected intersection of the selected piercing line with the fault +/- its uncertainty, and find the distance *along the fault* between the two projected piercing points. 

While many aspects of this manuscript have already been diligently improved, these issues with the measurement and presentation of *estimated offsets* (not "real displacement") remain problematic and should be addressed before publication.

Comments on the Quality of English Language

Review for English grammar clarity and consistency should be conducted before the next re-submission.

Author Response

Please see the attachment.

Author Response File: Author Response.docx