the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Evolution of submarine canyon-fan systems in fault-controlled margins: Insights from physical experiments
Steven Y. J. Lai
David Amblas
Aaron Micallef
Thomas P. Gerber
Hérve Capart
Abstract. Different fault settings make the morphology of submarine canyon-fan systems on active margins complex and diverse. In this study we explore the continuum of erosion, transport and sedimentation processes taking place in fault-controlled canyon-fan systems by using physical experiments and a morphodynamic model. Based on morphometric analyses we show how Hack’s scaling relationships exist in submarine canyons and fans. The DEM of differences (DoDs) demonstrate the growth patterns and allow to establish relevant relationships between volumes of canyons and their corresponding fans. We reveal strong self-similarities on canyon-fan long profiles and, through a new morphodynamic model, we capture their evolution over time, including the trajectory of internal moving boundaries. We observe that fault slip rate controls the merging speed of coalescent submarine canyon-fan systems and, when coupling fault slip rate with inflow discharge, a competitive influence arises. In this study we also uncover scaling relationships spanned from laboratory to field-scale. Overall, our findings are inspiring and valuable for field investigators and modelers to better interpret and predict the morphological evolution and sedimentary processes of submarine canyon-fan systems in active fault settings.
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Steven Y. J. Lai et al.
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RC1: 'Comment on esurf-2022-62', Adam Daniel McArthur, 14 Mar 2023
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Dear Authors,
I enjoyed reading this manuscript, which is a rare example of trying to model submarine sedimentation across an active fault scarp. The methodology of combining a sand-box model with flume tank experiments is novel and for this alone is worth publication, especially the supplementary videos, which would be great to include in the main publication if the journal has this capacity.
It’s mostly clearly written (with some noted exceptions in the Results and Discussion sections), with a logical flow and supported by good quality figures and explanation of the data and results. However, there are limitations in applying the modelling fine grained, diffuse sedimentation to the typically coarse grained deposits associated with canyons and hangingwall fans, which are typically dominated by mass-wasting. Particularly, if this work is to be used to “predict the morphological evolution and sedimentary processes of submarine canyon-fan systems in active fault settings”, which unfortunately, as written it does not. I outline my main concerns below; with some careful re-framing this could be published after major revisions.
I have annotated a PDF with a number of comments and minor corrections that will improve the final paper. The main points to remedy, which will improve the readers confidence in the work are:
- The study needs to be better framed, in terms of 1) modern work on source to sink systems (e.g., Sømme et al., 2009; Nyberg et al., 2018), 2) recent work on canyon systems (e.g., Bernhardt and Schwanghart, 2021; Soutter et al., 2021) and 3) the wealth of work on active margin systems (e.g., Bührig et al., 2022; McArthur et al., 2022). As it stands many of the motivations for doing this work seem dated and ignorant of the wealth of work on active margins over the past twenty or more years.
- This leads to some confusion on the actual features being modelled, which are really hangingwall fans (sensu Leeder and Gawthorpe, 1987), rather than classical submarine fans. This is an important distinction and the study needs reframing in this light. This has the added complication that most hangingwall fans are constructed by a mixing of turbidity currents and mass-wasting (e.g., McArthur et al., 2013; Barrett et al., 2021). This at least needs to be considered and discussed, otherwise the whole premise of the study seems flawed.
- Furthermore, the basic assumption that flows through canyons and that form said hangingwall fans are fine grained, here modelled as saturated brine to represent mud-rich turbidity currents, using a diffusion model really limits the application of this to understanding how natural systems evolve. We know that most active margin canyons are conduits for- and hangingwall fans build stratigraphy that is a mix of high concentration turbidity currents, debris flows and mass transport deposits. Furthermore, recent work has shown how varying grain size and sorting of flows strongly influences their ability to erode, bypass or deposit (Crisóstomo-Figueroa et al. 2021; Amy and Dorrell 2022). Therefore, the limitation of modelling fine grained particles with diffusion vs. the complicated nature of reality should at least be stated up front and discussed if the authors truly believe this work will help us “understand the initiation and evolution of fault-controlled submarine canyon-fan systems driven by downslope gravity flows”.
- Much is made of varying the effects of varying inflow discharge. However, upon seeing the supplementary videos, it appears that most of the sedimentation through your canyons to build the fans is actually via footwall erosion and reworking – this is actually closer to reality that your described methods and results. Much more description of this process, perhaps accompanied by greater investigation of this erosion would help address some of the concerns of how appropriate it is to use these experiments to understand natural systems. Further information on the composition of the footwall is required in the methods and this process needs to be incorporated into your Morphodynamic model.
- However, the flows shown in the supplementary videos do not appear to resemble the “long-lived hyperpycnal flows or mud-rich turbidity currents” that are claimed to be modelled. Particularly, the flows moving sediment and building your hangingwall fans appear as simple sediment gravity flows, i.e., grain flows / avalanches, which have very different flow properties to turbidity currents. This fundamentally questions the applicability of this study to natural systems dominated by turbidity currents.
- The Discussion needs to be just that. I.e., you should compare your results to those of other studies to discuss what works and what doesn't. At the moment it’s just a rather long winded attempt to further explain the results of your models. Breaking up the discussion into distinct sub-sections to address specific points will also help build a narrative.
- As it stands, this main conclusion and other broad statements about how this modelling can help us understand the geological evolution of submarine canyons and “submarine” (hangingwall) fans is not supported by your data or results.
- That is before even considering the multitude of natural complications that may arise e.g. other processes (i.e., canyon tides and internal waves, contourites etc.), variability of natural systems (e.g., seasonal, Milankovitch cycles etc.), variation in catchment area (size and composition), structural complexities (e.g., faults are rarely just one fault, but normally a fault zone of many structures active at different times with different effects; effects of earthquakes on landslides and sediment remobilisation, etc.) and many more besides. I appreciate these natural variabilities cannot be addressed by simple modelling, but they should at least be acknowledged if you want to suggest these results can be applicable to natural systems.
- One of the key things to consider in an attempt to reframe the impact of this work, is that your models are free from many of those complexities, e.g. climate change, variation in sediment supply. So with all things being equal it appears tectonics is the overriding control; this would be a very useful main conclusion and agree with work on natural systems (e.g., Soutter et al., 2021).
References above are all listed in the annotated PDF at the appropriate points.
If the authors are unsure of my comments, then feel free to contact me at a.mcarthur@leeds.ac.uk
Yours sincerely,
Adam D. McArthur
Steven Y. J. Lai et al.
Steven Y. J. Lai et al.
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