Not just any water, Watson

Dr. Watson arrives in front of the fireplace ahead of Sherlock Holmes and announces that everything is ready for a continuation of their discussion.

– I have prepared two Tequila Daisies for us, he announces.

What on earth is that, Holmes asks as he arrives.

– You probably have not heard about The Saber-Tooth Curriculum, Dr. Watson asks.

What on earth is that, Holmes asks again, even more bewildered.

– I take it, you never heard about J. Abner Peddiwell either, Dr. Watson continues.

Holmes grunts: Who on earth is that? Enough nonsense, let´s get on with some serious work.

We left off yesterday with some unsolved business, namely the counter-push extension of continental margins during rifting of South America and Africa, and we also touched upon the need to get mantle upwelling started in order to produce the ridge push, so let´s get started.

– Not even tasting the drink, Dr. Watson asks, slightly disappointed.

Holmes, already lecturing: No more time for nonsense, I said!

As evidenced by many, there is always a pre-history that might influence a geologic history of interest [1, 2]. This is also the case with South America and Africa. Before the rifting of the South Atlantic started 143 million years ago [3], they were joined together because the Congo craton collided 550 million years ago with what is now South America [4, 5].

In addition, even before that, these two continents might have been involved in rifting in an even earlier episode, leaving geologic features similar to those observed today. These features would naturally have been accreted but possibly not destroyed during the collision 550 million years ago.

The conclusion to all of this is that to interpret the present geologic status, you have to know something about the history of the region, long before the present. Continental margins have a tendency to be visited by tectonic processes, ever so often [6, 7].

One thing is certain though, the ocean or terrain separating the two continental plates had to disappear before the continental crusts of the Congo Craton and South America collided. A vast volume of oceanic crust had to be subducted and pushed deep into the mantle beneath the collision zone. As we have established before, more material than the upper parts of the lithosphere was – and is – always subducted. However, the most interesting part for our discussion is the part that contacted the sea and was invaded by it.

Due to mechanical properties and mineralogical processes within the subducting oceanic lithosphere, a lot of seawater also subducts into the lithosphere [8, 9].  Different clay minerals are formed during exposure of the lithosphere to seawater at moderate temperatures, even down to 25 km depth. The minerals carry water mostly in the form of hydroxyl groups or adsorbed water.

– OK, Holmes, we have touched upon this “water” in previous discussions. I assume that you have reasons for bringing it up again in the context of what we were discussing last evening, Dr. Watson manages to say before Holmes continues.

Holmes: Indeed, Watson, the trapped water and hydroxyl groups within different subducted minerals become unstable as temperature increases in the sinking slab. The OH-groups therefore escape to form H₂O – that is, water.

Holmes continues:  The diffusion of water from the subducting slab into the surrounding mantle lowers the melting temperature of the rock and is a prerequisite for the dissolution of minerals and the formation of new ones. These mobile melts/solutions are located between the mineral grains in the otherwise rigid mantle, thereby lowering the effective density and viscosity of the fluid-infected rock volume.

Many researchers interpret the existence of these fluid-infected rock volumes because of exposure to higher temperatures. They think that water released at these depths is squeezed out immediately or escapes quickly in volcanic eruptions of water-bearing melts. This is not the case. At great depths, water and rocks can form completely miscible and supercritical solutions, while higher up in the mantle, saline water can form mobile intergranular fluids that create permeable rock volumes, a permeability that is created by the pressure- and temperature conditions – and thus, the fluid phase with its dissolved minerals is not squeezed out [10, 11, 12].

Fluids and/or partial melts may continue to accumulate laterally in an increasing volume of ​​the mantle if the subduction process continues. Thus, the volume of mobile fluids trapped in the permeable mantle chambers can become very large. This leads to a reduction in the density and viscosity of the invaded mantle volume and will ultimately lead to uplift and faulting of the terrain above the rising mantle dome.

The mechanical properties (brittle/ductile and strength) of the hydrated mantle and overlying crust prevent rapid ascent all the way to the surface [13, 14]. This situation might therefore exist for millennia without much happening besides surface uplift. [15, 16].

For extended periods, the uplifted surface might be exposed to increased erosion due to its elevation, and some fluids might escape to reach shallower depths. This might be depths where temperature permits serpentinization of the upper mantle/ lower crust above the mantle dome. The presence of water at high pressure and temperature at these depths may lead to mineral reactions (metasomatism) in the upper mantle. A separation of mantle rocks into heavier and lighter phases may occur, leading to erosion of the lithosphere from below as the heavier rocks sink. [18, 19, 20] This process would allow the fluid-rich asthenosphere to rise further to replace the volume of downwelling material.

– Do you know a region where this situation has been observed, Dr. Watson asks.

Holmes explains: Indeed, I do. Look at the uplifted Utah, Great Basin – Colorado Plateau transition right next to an extinct subduction zone, 1.7 billion years old. [20]. In this region, signs of asthenosphere upwelling as well as downwelling are observed in a mountainous region where saline fluids are leaking to the surface via deep faults, indicating that several of the prescribed elements are present.

Typically, the uplift prior to rifting has been observed to be on the order of 3-4 km due to such processes [18, 21]. This is almost double the uplift observed along mid-ocean ridges, suggesting that the initial uplift forces caused by «wet» upwelling of the asthenosphere are large and may reach a sufficient maximum for rifting to allow fluids to escape. After the escape of the fluids, we are back to uplift and upwelling due to thermal dis-equilibrium alone.

– OK, Holmes, Dr. Watson states, to sum up:

– Just before rifting, the fluid-rich asthenosphere has caused the removal of the upper mantle lithosphere, it has created a serpentinized zone in the lower crust/upper mantle, allowed by ascending fluids at sufficiently low temperatures – that is, ca. 500 degrees C. It has lifted the surface in the affected region by several km, thereby causing some faulting and increased erosion/redeposition. In addition, since the asthenosphere has to go somewhere, it is pushing the serpentinized lithosphere laterally.

– However, Holmes, I have one question.

Holmes replies: Fire away, Watson.

– With all this water and all this heat from the rising asthenosphere, why are we not seeing massive volcanism in cases such as this, Dr. Watson asks.

Holes says: Ah, Watson, I am so glad that you brought this up. Thermodynamic processes related to the vast amounts of water come into play. Moreover, it is a fact that certain facts are overlooked by many. You see, Watson, water has up to four times the heat capacity of common rocks [22, 23]. The implication of this is that a major part of the energy contained in the upper, wet, upwelled asthenosphere is carried by water. In addition, under conditions present in the lower/middle crust, water may undergo phase separation and even boiling.

When this happens, the heat contained in water will escape with the escaping fluids. Water can expand dramatically during phase change and convert its internal energy to volume while simultaneously providing heat to its surroundings as it ascends. This literally takes the heat out of the system without producing magma. Other effects related to water are also contributing [24]. However, we will not diverge any further.

Holmes concludes: You might compare the situation with what happens with a kettle of water on a gas stove. Regardless of the intensity of the flame, as long as there is water in the kettle, the bottom of the kettle and the water inside it will remain at the boiling point of water. However, when the water is gone, you might even melt the kettle.

– OK, Holmes, Dr. Watson injects, I must sum up again: When the wet asthenosphere has reached a certain elevation and produced lots of serpentinites above, all hell breaks loose, and fluids start to escape to the surface after being subjected to phase changes and presumably large changes in the pressure experienced within the involved fluids and rocks. At what depth do you need to lift the system to produce this release of fluids?

Holmes replies: That is a good question, Watson. To produce the abrupt phase change, the system must breach the lithostatic pressure exerted by the overlying rocks. This becomes more likely when the rocks above are brittle and have cracks in them, or are capable of forming fractures under stress. When this is the case, our system might go from being pressurised by overlying rock masses to hydrostatic/fluid dynamic pressures caused by escaping columns of fluid. The depth that allows this is dependent both on temperature and mineralogy in overlying rocks, but in reality, around 15 km depth due to the weight of an escaping fluid column at actual composition, pressures, and temperatures.

– Might I assume that the serpentinites are playing a role in allowing our scenario to unfold, Dr. Watson asks.

Holmes bursts out: Exactly, Watson! This is when the true importance of the serpentinized lower crust is revealed. Because, Watson, serpentinites are very special because they may show brittle behaviour under conditions where rocks are normally ductile (500 degrees C, 10,000 bar) [25]. The implication of this is that the actions by the upwelling asthenosphere start creating long detachment faults in the serpentinite layer. Pressurised from below by fluids trying to escape, the detachment faults in the serpentinite become conduits for such fluids, allowing them to reach elevations where different, colder, and brittle crustal rocks allow their rise further up.

By the way, Watson, since many geologists, for some reason, are unable to envision serpentinizing fluids ascending from below, they get stuck trying to explain how this process could be sustained by water descending from the sea above. That is another story not to waste time on, Holmes concludes.

Holmes continues: We have to finish our explanation, Watson. As the serpentinites are stretched laterally and faults become lubricated by fluids, they gradually transmit the lateral forces to the upper crust. As the first chasm opens in the upper crust, a potential relief of several km is exposed at the point of maximum stress above the rising asthenosphere, carrying the uplifted crust. This is when the upper crustal elements start sliding towards the chasm, aided by gravity on top of the fluid-lubricated layers underneath.

By the way, Watson, a similar, fluid-lubricated system can be observed in the Kara-Bogaz-Gol, where a mega-landslide is occurring at slope angles of only a few percent [26].  An interesting feature of this landslide is that all the moving crustal blocks seem to move more or less simultaneously, and not one after the other. This prevents a former released block from being hit and observably deformed by a later arriving block.

Also keep in mind, Watson, that as soon as the fluids escape from a section of the upwelling asthenosphere, this section sink in due to the loss of volume in the form of fluids and dissolved solids. This increases the slope locally at the crustal base in the direction of where the fluid expulsion is occurring.

Anyway, as long as there is a slight downhill path towards the location of the initial splitting of the upper crust, crustal segments might slide in one direction while at the same time the entire plate segment is pushed in the opposite direction by forces acting beneath the serpentinite. The extent of sliding crustal blocks will mostly be determined by the extent of the serpentinite layer, which also expresses the regional extent of serpentinizing fluid supply from the mantle.

– Could you say more about the thermodynamic and mineralogical aspects related to fluid movements in space and time, or the fate of mass/volume ascending from the deep and ending up on the surface, Dr. Watson asks.

Holmes: Not tonight, Watson, the consequences of the often-unrecognised, non-volcanic, vertical mass movements, their influence on topography/stratigraphy, as well as the changing viscous properties of solutions/rocks/melts due to changes in pressure and temperature during ascent, are all highly relevant. However, those and related matters are subjects for another occasion.

Holmes admits: Now, Watson, please explain the idea behind the tequila drinks and the reason for bringing up the tiger story. I fail to see the relevance.

– Well, Holmes, the story in question [27] and its relevance, unfortunately, are subjects for another occasion, Dr Watson concludes, leaving Sherlock Holmes with a disappointed expression on his face.

– However, I might reveal that all the questions you did discuss, as well as the questions you did not discuss, are rarely treated as one subject. Maybe there is something wrong with our universities. This is where the book comes in. Enjoy, Holmes!

Dr. Watson finally gets the last word.

HANS K JOHNSEN

Inspired by Arthur Conan Doyle

References

1. Erdős, Z., Buiter, S. J. H., and Tetreault, J. L.,2025, The influence of accretionary orogenesis on subsequent rift dynamics, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-1065.
2. Manatschal, G., Lavier, L., Chenin, P., 2014, The role of inheritance in structuring hyperextended rift systems: Some considerations based on observations and numerical modeling, Gondwana Research, http://dx.doi.org/10.1016/j.gr.2014.08.006
3. Pichel, L. M., Legeay, E., Ringenbach, J.-C., & Callot, J.-P., 2023, The West African salt-bearing rifted margin—Regional structural variability and salt tectonics between Gabon and Namibe. Basin Research, 35, 2217–2248. https://doi.org/10.1111/bre.12796
4. Pisarevsky, S. A., Murphy, J. B., Cawood, P. A., Collins, A. S., 2008, Late Neoproterozoic and Early Cambrian palaeogeography: models and problems, In: West Gondwana: Pre-Cenozoic Correlations Across the South Atlantic Region, Eds., Pankhurst, R. J., Trouw, R. A. J., de Brito Neves, B. B., de Wit, M. J.,  https://doi.org/10.1144/SP294.2
5. Frimmel, H.E., 2009. Configuration of Pan-African orogenic belts in Southwestern Africa. In: Neoproterozoic-Cambrian tectonics, global change and evolution: a focus on southwestern Gondwana. Developments in Precambrian Geology, Eds: Gaucher, C., Sial, A.N., Halverson, G.P., Frimmel, H.E., 16, Elsevier, pp. 1452151. SSN 0166-2635, DOI 10.1016/S0166-2635(09)01610-7
6. Wilson, J. T., 1966, Did the Atlantic close and then reopen? Nature, 211, 676-681
7. Buiter, S.J.H., Torsvik, T. H., 2014, A review of Wilson Cycle plate margins: A role for mantle plumes in continental break-up along sutures?, Gondwana Research, Volume 26, Issue 2, Pages 627-653, ISSN 1342- 937X, https://doi.org/10.1016/j.gr.2014.02.007.
8. Worzewski, T., Jegen, M., Kopp, H., Brasse, H., Castillo, W. T., 2011, Magnetotelluric image of the fluid cycle in the Costa Rican subduction zone. Nature Geosci 4, 108–111. https://doi.org/10.1038/ngeo1041
9. Cai, C., Wiens, D.A., Shen, W., Eimer, M., 2018, Water input into the Mariana subduction zone estimated from ocean-bottom seismic data. Nature 563, 389–392 (2018). https://doi.org/10.1038/s41586-018-0655-4
10. Manning, C. E., 2004, The chemistry of subduction-zone fluids. Earth Planet. Sci. Lett. 223, 1-16. Earth and Planetary Science Letters. 223. 1-16. 10.1016/j.epsl.2004.04.030.
11. Holness, M., 2005, Melt-Solid Dihedral Angles of Common Minerals in Natural Rocks. Journal of Petrology. 47. 10.1093/petrology/egi094.
12. Hudson, T. S., Kendall, J-M., Pritchard, M. E., Blundy, J.D., Gottsmann, J.H., 2022, From slab to surface: Earthquake evidence for fluid migration at Uturuncu volcano, Bolivia, Earth and Planetary Science Letters, Volume 577, 117268, ISSN 0012- 821X, https://doi.org/10.1016/j.epsl.2021.117268
13. Huang, Y., Nakatani, T., Nakamura, M. et al. Saline aqueous fluid circulation in mantle wedge inferred from olivine wetting properties. Nat Commun 10, 5557 (2019). https://doi.org/10.1038/s41467-019-13513-7
14. Kawamoto, T., Yoshikawab, M., Kumagaia, Y., Mirabueno, Ma. H. T., Okuno, M., Kobayashi, T., 2013, Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab, PNAS, 110, No. 24, 9663–9668.
15. d’Almeida, Gérard A. F. A., 2010, Structural evolution history of the Red Sea Rift. Geotectonics. 44. 271-282. 10.1134/S0016852110030052.
16. Avigad, D., Gvirtzman, Z., 2009, Late Neoproterozoic rise and fall of the northern Arabian–Nubian shield: The role of lithospheric mantle delamination and subsequent thermal subsidence, Tectonophysics, doi:10.1016/j.tecto.2009.04.018
17. Göğüş, O.H., Sundell, K., Uluocak, E.Ş. et al.Rapid surface uplift and crustal flow in the Central Andes (southern Peru) controlled by lithospheric drip dynamics. Sci Rep 12, 5500 (2022). https://doi.org/10.1038/s41598-022-08629-8
18. Esedo, Raphael & Wijk, Jolante & Coblentz, Dave & Meyer, Romain. (2012). Uplift prior to continental breakup: Indication for removal of mantle lithosphere?. Geosphere. 8. 1078-1085. 10.1130/GES00748.1.
19. Magni, V., Király, A., 2020, Delamination, Reference Module in Earth Systems and Environmental Sciences, Elsevier, ISBN 9780124095489, https://doi.org/10.1016/B978-0-12-409548-9.09515-4.
20. Wannamaker, P. E., et al., 2008, Lithospheric dismemberment and magmatic processes of the Great Basin – Colorado Plateau transition, Utah, implied from magnetotellurics, Geochem. Geophys. Geosyst., 9, Q05019, doi:10.1029/2007GC001886
21. Issachar, Ran & Haas, Peter & Augustin, Nico & Ebbing, Jörg. (2024). Rift and plume: a discussion on active and passive rifting mechanisms in the Afro-Arabian rift based on synthesis of geophysical data. Solid Earth. 15. 807-826. 10.5194/se-15-807-2024.
22. Xiaoqing, S., Ming J., Peiwen, X., 2018, Analysis of the thermophysical properties and influencing factors of various rock types from the Guizhou province, E3S Web Conf. 53 03059, DOI: 10.1051/e3sconf/20185303059
23. Water Vapor – Specific Heat vs. Temperature Specific heat of Water Vapor – H2O – at temperatures ranging 175 – 6000 K. https://www.engineeringtoolbox.com/water-vapor-d_979.html

24. Popa, R-G., Tollan, P., Bachmann, O., Schenker, V., Ellis, B., Allaz, J.M., 2021, Water exsolution in the magma chamber favors effusive eruptions: Application of Cl-F partitioning behavior at the Nisyros-Yali volcanic area, Chemical Geology, Volume 570, 120170, ISSN 0009-2541, https://doi.org/10.1016/j.chemgeo.2021.120170

25. French, M.E., Hirth, G., Okazaki, K., 2019, Fracture-induced pore fluid pressure weakening and dehydration of serpentinite, Tectonophysics, Volume 767, 228168, ISSN 0040-1951, https://doi.org/10.1016/j.tecto.2019.228168.
26. Aslan, G., De Michele, M., Raucoules, D. et al. Transient motion of the largest landslide on Earth, modulated by hydrological forces. Sci Rep 11, 10407 (2021). https://doi.org/10.1038/s41598-021-89899-6
27. Peddiwell J.A., 1939, The Saber-Tooth Curriculum, McGraw-Hill Inc., New York, ISBN 07-049151-8.