Comments on the submitted pre-print “ Emplacement age of the Sevier Gravity Slide, Utah, USA” by T. Rivera, et al. 

Reviewer: Martha Papadopoulou

General comments: the study is looking into the association between gravity slides, their cause of origin (e.g. new injected magma into a magma chamber) and their consequences on the volcanism of an active volcanic region (e.g. decompression melting due to displacement) using age constraints from different crucial layers of the studied formation. The studied area is the Sevier Gravity Slide, and their results are compared with other studies. The manuscript reads well, giving insights on the geological history of the area, the work that has been done so far, and how this study connects with previous works on the area. There is an overall good understanding of the associations between the different layers and a good understanding of the methods used and their limitations. I really enjoy how the works builds upon previous results and refines them, adding to the knowledge of the volcanic history of the region, and to the poorly understood causes of a gravity slide as well as their consequences to an area.

Specific comments: the few comments provided here on specific issues identified are meant in order to improve the quality of the manuscript and help a reader with no previous knowledge of the studied area to gain a better understanding. It is upon the authors to decide if they will act on them or not, but hopefully they will be taken into account on a revised version.

1. Section 2 provides a lot of information about the geological background of the area, referring to a lot of features, such as plateaus, calderas etc.. I believe the section (and consequently the reader) would benefit from an extra map (or set of maps if too complicated) showing the location and association between these features. For example, maybe a map showing the extent of the Colorado Plateau and the Basin & Range province, with the Marysvale VF and the Marysvale gravity slide complex depicted on it; the location of the three calderas mentioned in the text; and the relative location of the Markagunt and Black Mountains gravity slides compared to the Sevier gravity slide. 
2. At the beginning of section 3 different units are described as deformed if within the SGS or undeformed if above it. However, towards the end of the section the Osiris tuff, which is marked as an undeformed unit, is described to be deformed but not from the SGS. I feel this is a bit contradictory. Maybe an additional sentence at the beginning (or somewhere else) of the section clarifying that all units deformed by an event other than the SGS will be seen (for the purposes of this study) as undeformed might be more appropriate.
3. What units does the Mount Dutton Formation comprises of? From the stratigraphic column in Fig. 1 I had the impression that Mount Dutton Formation includes all the various units depicted in there, then I got confused from Fig. 5, where Kingston Canyon Tuff and Mount Dutton Alluvium where described as belonging to separate formations? Could you add to the stratigraphic column the various formations and their extent?

Technical comments:

1. Not sure it’s only on my computer, but rows 44-46 (first three sentences in the introduction part) have a different format to the rest of the text. Please format accordingly
2. Rows 50-51 “…a friction induced… in sliding”: references needed
3. Row 54: could you define the word “gigantic”? E.g. average area extent
4. Rows 61&64: you have performed 40Ar/39Ar dating, why is there suddenly a * to the 40Ar? It’s not consistent with further down the text
5. Rows 81-82 “The breakaway regions… volcanic activity”: references needed
6. Row 84 “…the breakaway of the… batholith”: references needed
7. Row 86: for consistency with the rest of the text provide first the older then the younger age constraint of the volcanic activity (32-23 Ma)
8. Row 86: please re-write to “Peak volcanic activity (32-23 Ma)…”
9. Row 117: Fig. 2a-c
10. Row 118: “…and biotite (Fig. 2c)”
11. Row 118: “…igneous rocks (Fig. 2b)”
12. Row 120: Fig. 2d-f
13. Row 121: “…1-3 cm long (Fig. 2e)”
14. Row 122: Fig. 2i
15. Row 131: Fig. 2g-h
16. Row 221: ages 23.0 and 25.4 presented as advised in comment 7
17. Row 225: show the Buckskin Breccia the stratigraphic column of Fig. 1

General Comments:

The study presented in this manuscript utilised structurally constrained samples and multiple geochronometers to tightly constrain the emplacement age of the Sevier Gravity Slide. Utilising this approach the authors contribute to the ongoing debate around the causality of gravity slides. The manuscript is structured well, the data is presented logically and clearly, with sufficient consideration and use of previous data/studies results, and connects these localised results well to the ‘outstanding questions’ of the origin and connections between gravity slides and igneous activity. I thought the approach of the study is well informed of and builds upon previous work to then have a targeted approach of what techniques and samples are best to address the hypothesis. The understanding of the geochronology data within the context of the structural relations of the gravity flow deposits provides a cool approach to provide such tight constraints in timing between these various units.

Specific Comments:

1. As the study area is quite a local study, additions to the map figure would be much appreciated by a reader such as myself without much knowledge of the geology of the area/region. If there is the ability to have more panel slightly zoomed out further than the presented zoom (still including this panel though) would benefit the readers who are not familiar with the local geology, as well as providing easier ability for the readers to follow the large number of features presented in the ‘Introduction’ and ‘Geologic background’ sections.
2. I would like to see just a little more detail added to the analytical sections 3.1 and 3.2. As a few different analytical techniques were conducted for the Ar-Ar geochronology (some of the samples (the ash flow tuffs) are weighted means of many single grain analyses, then the lava and pseudotachylyte are step heated), the section would benefit from the added information of how the grains were selected. Were the sanidine and the plagioclase also hand-picked prior to the analyses like the pseudotachyle glass? Where these selections made under a binocular scope? Were the two step heating experiments on populations? Although some of this may be known/be able to be inferred by readers with an Argon geochronology background, having this information detailed would help the interpretations/understanding the interpretations being made of the data. Just about the same request of added detail to the U-Pb analytical section as well; were the zircon grains picked after being separated via panning, heavy liquid, and magnetic, or is this more of a dump mount after the various separation techniques? This information will provide more clarity on the assessing the interpretations being made of the U-Pb data (hand picking can provide some bias, therefore good to note particularly as multiple fractions (with significant peaks) are observed in the Kingston Canyon Tuff (images of the analysed grains in the supplementary material would be great as well).

Technical Corrections:

Line 61: The asterisk is not represented in every instance, could the ‘*’ after 40Ar be defined… and if required, the writing of 40Ar*/39Ar (instead of 40Ar/39Ar) be standardised throughout the text. Those that are not as familiar with the argon system may not immediately understand the use of the asterisk as a radiogenic Ar notation.

Line 99: The Markagunt gravity slide and the Sevier gravity slide have already been defined as MGS and SGS, respectively, in the ‘Abstract’ as well as lines 54 and 55 in the ‘Introduction’. No need to define them again here.

Line 195: Same comment as above regarding the use of the asterisk after 206Pb.

Figure 5 Caption: IsoPlotR should be IsoplotR. The two references in the caption are not in the bibliography; Galbraith and Laslett (1993) and Vermeesch (2021).

Figure S2 Caption: A space needs to be added after ‘(b)’.

Supplementary Material Tables 2-5: The Taylor (1982) and the Min et al. (2000) references are not included in the bibliography.

Supplementary Material Tables 6-7: Could a ‘caption’ for the U-Pb tables be added that denotes the uncertainties (e.g. are the isotopic ratio data 1 sigma %, 2 sigma % or 1 standard error %).