Response to Reviewer 1 Comments

General Comments

1. This manuscript by Orr et al. builds on a rich history of quality work done by this group tackling similar problems across the NW Argentinean Andes with the aim of using the rich geomorphic and tectonic settings of the region to constrain landscape responses and timing to climatic and tectonic perturbations. The paper is well written, reasoned, cited, and beautifully illustrated across all the figures. I really appreciate the care that this group of authors has taken with this work and think it should be accepted. The manuscript builds on previous work and datasets in a rigorous way to provide evidence for the preservation of varying geomorphic periodicities and processes within the same basin. I think the authors should be proud of their work. 

Thank you very much for your through review and constructive feedback on our manuscript. We have carefully addressed all of your comments, clarifying a number of aspects of the project. We believe that these adjustments have helped us to improve the manuscript – thank you!

1. The only weakness I find in the work is inherent to the methods and certainly no fault of the authors. Exposure dating of surfaces is just difficult, and even when you are as careful as these authors, there are just so many unknowns that can affect the CRN ages. This is especially apparent with the G1 fans. The authors rightfully acknowledge the messiness of these data and derive an incision rate from 800 ka to 500 ka that they argue is related to the MPT. The problem is that you have to explain away a lot of ages either through inheritance for the older ones or erosion for the younger ones. This in turn leaves you wondering which ones are reliable and how do you know? The authors state in line 516 that they “suggest that the G1 fan record can instead be used to capture an extended phase of net incision within the Sierra de Pascha tributaries.” Given the uncertainty in the ages in general it is hard to say that this incision occurred over 300 kyr or over 20 kyr. The point being that they invoke the MPT but without real good constraints you could invoke any number of drivers of incision over that period of time.

Thank you for your feedback. Working with an age dataset with such broad age distributions is a challenge. By resampling the Qf_1 depth profile and integrating it with the new boulder ages, we were able to constrain the age of the oldest G1 fan surface. This then served as a benchmark for the rest of the dataset, enabling us to interpret the remainder of the fan record with increased confidence. Based on the fan stratigraphy and our observations of the surfaces and boulders (see Supplement 2 and 3), we identified the boulders which were likely affected by erosion (TB19_06, TB19_08, TB19_14) or inheritance (TB19_02, TB19_04). This enabled us to constrain a phase of net incision (ca. 800- 500 ka), during which each of the G1 fans were active at some point, before being abandoned. As stated in the manuscript, applying ²¹Ne to some of these surfaces may help to reveal the complexities in burial history.

We acknowledge that there remain some uncertainties about the ca. 800-500 ka net incisional phase. Importantly, we are not suggesting a period of continuous incision during this time. Given our confidence in the Qf_1 dating (and that the other surfaces broadly fit into the a. 800-500 ka window), we argue that the G1 fans nicely capture a phase of net incision, before a period of lower geomorphic activity. The onset of G1 fan activity and abandonment appears to coincide with the MPT. We believe that the depth profile results are robust and support the idea of the onset of rapid incision around 800 ka. How fast it proceeded is less clear, but some preliminary ²¹Ne data from a new ongoing project (https://doi.org/10.5194/egusphere-egu24-14778) suggests that the incision was faster than 100-kyr cycles would imply. We feel as though this argument is compelling because similar phases of net incision linked to the MPT are recorded throughout the Andes. Throughout the manuscript, we acknowledge that there are other drivers during this time that may have contributed to the net incision. However, we have revised the wording in this section to clarify our arguments.

L534: ‘Given these complexities in the fan chronostratigraphy, rather than identifying discrete phases of aggradation and incision for each fan surface, we suggest that the G1 fan record can instead be used to capture a phase of net incision within the Sierra de Pascha tributaries. Crucially, this is unlikely continuous incision, but rather phase of net incision, which was punctuated by the formation of individual surfaces, possibly controlled by higher frequency climate cyclicity (e.g. 100-kyr). If so, this would imply periods of faster incision through the fill’.

1. My only recommendation would be to temper the wording a bit (things like “likely”) with respect to large-scale climate drivers as there is a fair amount of ambiguity that results from this technique and the correlations between fan age climate proxies in Figure 7 are not always obvious. I would tend toward phrases like “we propose” as the authors have done in their key finding #2 in the conclusions and acknowledge that there is considerable ambiguity. 

Thank you for the recommendation. As advised, we have adjusted the wording slightly to acknowledge remaining uncertainties in the results. We feel that Section 5.3 approaches this argument cautiously and draws on evidence from other studies to suggest a regional event. Rather than saying that the MPT has driven the incision entirely, we argue that there is compelling evidence of a potential link between them. We also acknowledge that other drivers (e.g. tectonic activity) may also contribute here. We have adjusted a couple of paragraphs to emphasise that we are being cautious with our interpretations.

L754: ‘Enhanced incision likely linked to the MPT has also been recognised at other locations in the Central Andes (Fig. 1A), including the Casa Grande Basin (23°S) in the Eastern Cordillera, the Salinas Grandes Basin (23.5°S) of the Puna Plateau (Pingel et al., 2019b), and the Iglesia (30.5°S) and Calingasta (32°S) basins in the Western Precordillera (Terrizzano et al., 2017; Peri et al., 2022).’

L770: ‘While it is not possible to discount a tectonic influence on landscape change in the upper Toro basin entirely due to some chronological ambiguity in the datasets and inherent challenges of deconvolving different forcing mechanisms, the links between MPT climate and incision, and its expression elsewhere in the Andes and beyond, is compelling.’

Specific Comments:

1. Any hypothesis why there is such a discrepancy between boulder dimensions between G1 and G2? Do the authors think this is systematic (source area difference?) or just a sampling bias? Is it evidence for widespread erosion of the G1 boulders?

The G1 fan surfaces appeared more weathered and eroded (with some boulder spallation) than G2. This is expected, given that the surfaces could be over ca. 400 kyr older and likely explains why the G1 boulders were noticeably smaller than those on G2. The boulder size is unlikely to reflect a difference in source area, but might relate to the hillslope processes (or even the size of debris flow) that transported them. We made a concerted effort to sample large, tabular, stable boulders that stood tall of the fan surface, with minimal evidence of weathering and surface spallation. While erosion may have affected some of the boulders (see response to Q2), our careful sampling strategy means that we can be more confident that the boulder ages reflect periods of surface activity and/or stability.

Nevertheless, we are finding that the ²¹Ne data, in combination with ¹⁰Be data for individual boulders, can help to constrain boulder erosion rates (which, so far, are extremely low). We are looking forward to seeing whether those data give more insight into the differences in boulder sizes, but those data and accompanying analysis go beyond the scope of this manuscript.

1. Figure 6 - I guess part A is informative for the reader but any comparison with the fan record in C is sort of laughable. Maybe consider what it adds to the story.

Thank you for the feedback. We believe that it is important to include this data in the figure as it reinforces our argument that assigning G1 surfaces individually to particular forcing or events is not appropriate. Fig. 6 also includes some important geomorphic and isotope data which is used to explore some of the alternative drivers of incision. On the whole we think this figure is appropriate, and helps to capture some of the climate, tectonic and geomorphic history of the basin on these very long timescales.

1. Is it possible to put glacial extent in the Pascha headwaters on Figure 1B since you discuss local glaciations on line 592? Not sure but it looks like Martini et al., 2017 and Luna et al., 2018 could be put in there and give some spatial context for this part of the discussion.

The past glacier extents in the tributaries have not been mapped or dated, although this presents an exciting avenue for future research. While the studies by Martini et al. (2017) and Luna et al. (2018) offer excellent glacier chronologies for local basins, we are hesitant to extrapolate their data and apply their observations to the Toro Basin. Instead, we suggest that in line with regional glacial records (D’Arcy et al., 2019a), the Pascha tributaries were likely glaciated for periods in the last ca. 100 kyr.

1. Figure 8B - You say “no significant geomorphic change recognized in the lower basin” but it appears in the diagram that there has been considerable mainstem Rio Toro aggradation from panel A to B. I think it is important to also detail what is going on along the mainstem in this figure as this is creating the local baselevel and presumably propagating any climatic signal between the two.   

Thank you for catching this error! The terraces record significant aggradation in the lower basin at this time (as reflected in the figure and later discussion). The local base level rose in line with this aggradation. The phrase "No significant geomorphic change in the lower basin" refers to the absence of an incision record. This has been clarified in the figure caption.

L605: ‘Aggradation was recorded in the lower basin (Tofelde et al., 2017).’

1. Line 693 - Doesn’t this fall in a drier period based on the panel A in Figure 7? It is a period of lower insolation which seems to correlate with warmer sea surface temp and less negative delta 18O. Have a look and see if you agree. 

Thank you for catching this omission. We have adjusted this statement to reflect your comments.

L723: ‘This points to a modest phase of net incision in several Sierra de Pascha catchments during a dry interglacial period (Fritz et al., 2007).’

1. Conclusion point 3 - I don’t understand the data that is backing up the statement “the abandonment of the G2 fans is restricted to glacial periods”. In Figure 7 QF_5 and QF_6 both have pdfs that mostly predate any glacial ages. Is the idea that the onset of glaciation drove abandonment of these?

Thank you for this comment. D’Arcy et al. (2019a) argue that glaciation is this region broadly in phase with insolation cycles, periods of SASM strengthening and/or northern hemispheric events. While there are no glacial records local to Toro that recognise a glacial event between 70 and 60 ka, the climatic conditions were sufficient to sustain glaciers. Glaciers were also recorded in the Central Andes at this time (D’Arcy et al., 2019a).

1. Also what constrains the very long tail of the glacial PDF if there are no points out there? Is that curve taken from a more extensive regional study? You should state in the caption what constrains this PDF. 

For clarity, we have removed the long tail on the glacier PDF.

1. Conclusion point 5 - I think it needs to be pointed out in section 5.4 that the Rio Iruya dataset is different from the others in Figure 9 because it is not measuring the timing of formation or abandonment of a geomorphic feature but rather a nearly continuous chemical sediment signal of erosion from a catchment. I am still wrapping my head around what it all means in terms of comparing them but I think it is an important distinction to make.    

Thanks for highlighting this and we agree that this is an important distinction to make. We have added this clarification in the Fig. 9 caption.

L777: ‘Unlike the other records of aggradation and incision, the Iruya record is derived from the basin's sedimentary record and is a paleo-erosion dataset.’

Technical Corrections:

1. Line 520 - You say ~0.07 mm/yr of incision between 800 ka and 500 ka but then at line 557 it is 0.8 mm/yr. Just need to decide on the best value and be consistent.

Thank you for catching this error. This incision rate is now quoted throughout the manuscript as 0.7 mm/yr.

1. Figure 9 - Fisher et al, 2016 should be Fisher et al., 2023 in the caption or the citation needs to be added for the 2016 AGU talk. The 2023 article is better to cite than the 2016 AGU talk though.

The Fisher et al. (2023) publication is now cited throughout the manuscript.

1. Fisher et al, 2023 reference is missing the last author in the reference. Should be… , and Lourens, L.J.

The reference has now been revised to include the last author.  

Response to Reviewer 2 Comments

1. Congrats on this interesting study. Here are some minor comments meant to improve this work in the paragraphs related with the geological setting.

Thank you very much for reviewing our manuscript and providing some helpful feedback on the sections relating to Toro’s geological setting. We have addressed each of your comments and made the necessary changes to the revised manuscript.

1. Line 123: I wonder what DeCelles et al., 2011 doing here, as they have not studied nor mentioned the course and nature of the Rio Toro .

Thanks for catching this error. We have removed the citation.

1. Lines 129-132: possibly site primary literature, or indicate by „reviewed in“

Thank you for the suggestion. We have added the Alonso (1992) citation, which helped to inform the adapted geological map in Fig. 2A. 

Alonso, R.N. Estratigrafía del Cenozoico de la cuenca de Pastos Grandes (Puna Salteña) con énfasis en la Formación. Revista de la Asociación Geológica Argentina, 47(2), 189-199.1992.

1. Lines 132-134 „The Solá fault has been active since at least the Pliocene, and tectonic deformation from the Miocene to mid-Pleistocene has been recorded along the San Bernardo and Gólgota faults (Marrett and Strecker 2000).“ Comment: ca. 10-9 Ma growth strata in the Agujas Cgl. in the Qda Cerro Bayo, along the Sola Fault indicate fault activity since at least Late Miocene times. Similarly, pre-13 Ma growth strata within the Barres Sst along the Golgota Fault set the minimum timing for deformation in the area (see DeCelles et al., 2011; Pearson et al., 2013; Pingel et al., 2020). 

Thank you for highlighting these omissions. We have adjusted the paragraph as recommended. We have also added the additional citations.

L135: The Solá fault has been active since at least the Late Miocene, and tectonic deformation from the Miocene to mid-Pleistocene has been recorded along the San Bernardo and Gólgota faults (Marrett and Strecker 2000; DeCelles et al., 2011; Pearson et al., 2013; Pingel et al., 2020). The Gólgota fault reactivated after ca. 0.98 Ma (Hilley and Strecker 2005).

1. Lines 157-159: (1) There is no major N-trending anticline separating the Río Toro from the fan deposits, as indicated here in the text and in the geological map in Fig. 2. The topographic expression causing this separation is formed by west-tilted basin strata (Barres; Agujas; volcanics of the Las Burras-Almagro-El Toro magmatic complex; and Alfarcito Conglomerates), likely deformed via basin-internal shortening in response to contraction along the Golgota fault. The map in Fig. 2 itself shows no repetition of the strata, only westward younging, as would be expected for W-tilted strata.

Thank you very much for your comment. We have carefully reviewed the geologic maps of the basin and agree with your argument. We have therefore removed the mapped anticline from Fig. 2A and adjusted the manuscript text.

L163: ‘The Middle Miocene Barres Sandstone and Agujas Conglomerates, interbedded with lava flows, and the Pliocene-Pleistocene Alfarcito Conglomerates make up the west-tilted strata, which lay between the fan deposits and the Río Toro (Fig. 2A; Hilley and Strecker, 2005; Mazzuoli et al., 2008).’

L616: ‘To elaborate on the first possibility, the Sierra de Pascha catchments are positioned behind and perpendicular to west-tilted and deformed basin strata (Barres Sandstone, Agujas and Alfracito Conglomerates, lava flows) (Fig. 2A). In concert with the work by Hilley and Strecker (2005), we suggest that channel incision through the resistant sedimentary units accelerated sometime between 0.98 and 0.8 Ma. Once this incision propagated upstream, the removal of weakly consolidated sedimentary units in the upper basin was likely efficient (Hilley and Strecker, 2005). This evolving topography could therefore help to explain the net incision needed in the upper Toro Basin to preserve the alluvial fan surfaces between ca. 800 to 500 ka, and why terrace levels in the lower basin are not recognised during this time interval.’

1. Something is wrong with the Mio-Pliocene basin stratigraphy: The Agujas Conglomerates are missing entirely from the record. The middle Miocene Barres sst is overlain by the Late Miocene Agujas Cgl, followed by the Alfarcitos cgl. ; all units are intercalated with volcanics of various kind.

Thank you for catching this oversight. We have adjusted the legend of Fig 2A and included the Agujas Conglomerate in the main text.

L163: ‘The Middle Miocene Barres Sandstone and Agujas Conglomerates, interbedded with lava flows, and the Pliocene-Pleistocene Alfarcito Conglomerates make up the west-tilted strata, which lay between the fan deposits and the Río Toro (Fig. 2A; Hilley and Strecker, 2005; Mazzuoli et al., 2008).’Resistant Barres, Agujas and Alfarcito units characterise several erosional surfaces that stand ~700 m above the modern river channel.

1. The literature presented to underpin the commented statement is partly inadequate: a) DeCelles et al.’s part on the Toro Basin is from the Qda. Cerro Bayo along the Sola Fault near Est. Maury not discussing the upper Toro Basin; b) Robledo et al. is a paleontological work dealing with flora and insect trace fossils of a Mio-Pliocene section along the Sola Fault near Gobernador Solá; c) Mazzuoli et al., is the only reference that fits, as they have dated the volcanics in the center of the ridge. None of the sources presented mention an anticline.

Thank you for your comments. We have removed the relevant citations.

1. Line 171: „South American low-level jet (SALLJ)“ in Fig. 2 it is called LLAJ–Low-Level Andean Jet.

This has been changed in the revised manuscript.

1. 1: Label for Almagro Range is positioned wrong.

The position of the label has been adjusted.

1. 2: The legend states that the unit below the Miocene volcanics is the Barres Fm. This is not correct. It is mainly the Agujas Conglomerates plus the underlying Barres sandstone.

Thank you for catching this. We have adjusted the legend to be consistent with the geological map in Pingel et al., (2020) (Fig. 3).

Thank you very much for your thorough review and constructive feedback on our manuscript. We have carefully addressed all of your comments (see attached file), clarifying a number of aspects of the project. We believe that your feedback has helped us to strengthen the manuscript – thank you!