Sherlock Holmes and his dear friend Dr. Watson have retired after a superb dinner, to their favourite Chesterfields, facing the fireplace. Holmes with his pipe and tobacco within reach, both with a well-tempered Gran Duque de Alba (for a change) in their right hand.
– I can feel that something is going on in your head, Dr. Watson breaks the silence.
Holmes replies: Have you ever heard of the Hippopotamus in the basin?
– I have heard of the elephant in the room – are they related, Watson remarks.
Holmes: Yes – in a strange way – would you mind doing some experiments?
– You know I cannot resist experiments, Watson replies.
Holmes says: OK then – go get your long U-tube and some blue and red liquids.
– The only U-tube in the house is a meteorological instrument filled with dangerous mercury that is not to be played with – and I have no red and blue liquids, Watson replies.
Holmes: Well then – we will have to do a thought experiment instead and settle for some paper and a pencil – if you would be so kind.
A minute later Watson is ready to receive further instructions.
Holmes: Make a drawing of a large U-tube, 1 meter tall, standing vertically with both legs pointing upwards.
– The paper is too small. It is only 30 cm in its longest direction, Watson laments.
Holmes: The size of the drawing is not important. Make a scaled drawing then, just keep track of the scale.
A minute later Watson confirms that he is finished.
Holmes: Great! Now, fill the tube with an imaginary, blue liquid so that the level in each branch is at the 50 cm level. Imagine further that this blue liquid has a density of 3-something.
– OK, it`s done. Now what, Watson replies.
Holmes continues: Now, you must fill another liquid into the left branch of your U-tube. This liquid is red and immiscible with the blue liquid. The red liquid has a density of 2-something. Where do you anticipate the new level will be in the two branches if you fill in a volume that equals 3 cm in height within the tube?
– Well, I expect that the two branches will settle in some kind of force equilibrium – and thus 3 cm in height of density 2 in the left branch must balance 2 cm in the extra height of density 3 in the other branch, Watson replies.
Holmes (a bit annoyed): So, give me the cm-value for the new liquid levels in the two branches.
– Yes, I see what you mean. Well, we have filled in 3 cm more liquid – and the extra volume has to be somewhere. I would say that the left branch now has a liquid level of 52 cm and the right-hand side level is 51 cm. Let me check. 47 cm times 3, plus 3 cm times 2, equals 153. 51 cm times 3 also equals 153. YES – it is balanced and in equilibrium, Watson exclaims proudly.
Holmes: OK then, very clever. I have been fooling you a bit, this is all about geology.
– Geology, Watson exclaims.
Holmes: Yes, if you imagine the red liquid to be the crust and the blue liquid the mantle, what we have performed is an experiment where more crustal load has been put on the mantle in one location. Let each cm of liquids in the tube represent several kms in geology and you will see the relevance.
– Well, what is the point, I still have a problem seeing the relevance, says Watson, somewhat bored.
Holmes, trying to regain the attention of Dr. Watson: The point is that you cannot make a sedimentary basin by first loading crustal masses on top of a continental surface. Loading more crustal mass on top of a continental region will make the surface rise, not sink! You first need a hole into which sediments may flow.
– Well, every geologist knows that your mantle-U-tube experiment is not realistic at all. The mantle is so dense that it rarely lifts above 4-6000 meters depth in the ocean, and beneath continents, it is even deeper, Watson says.
Holmes: Not so quick my friend, if we load an equal amount of 30 cm of red liquid in both branches – or X km of rock in geology – would the relative change in levels between the two branches be different? No, it would not! Moreover, not in geology either.
Holmes: If we remove a bit of crust here and there from a large continent and place it all in one region, the surface of that region would rise relative to the rest of the continent. But, and this is an important BUT, the original surface beneath the new masses would sink to a lower level than initially as a response to the load and create the false impression of a subsided basin existing before we moved in the new masses! If the region in question were 100 m above sea level before the mass transfer, it would be even higher afterward.
Holmes: However, this is not the Hippopotamus in the basin – it is not even the elephant in the room. So how would you make a sedimentary basin now that you know this?
– How about tearing the continent apart and creating a sort of valley – a rift valley, into which sediments could be transported and fill the basin while making the bottom of the valley subside? Isn`t there one in Africa, Watson replies.
Holmes: That is a very good idea, but why would such a valley appear?
– Well, as you know – continental plates are torn apart due to tectonic forces, Watson answers, on the offensive now.
Holmes: Not so fast my friend, what would make a continent break up exactly at a certain location? Why does it break there and not “everywhere”, if it is pulled by its edges?
– I`m afraid you got me there Sir, I have not the faintest clue, Watson admits.
Holmes, again on the offensive: OK, Dr. Watson, – this is when the real fun begins.
Holmes: Let`s say that during the night while you are sleeping, I sneak in and fill in another liquid into the blue liquid in the left branch of your U-tube. This liquid is special: it is very light, equivalent to 0.5-something. Its properties will make it evaporate if it encounters atmospheric pressure. Now imagine that this new liquid is miscible with both the blue and the red liquid and that I inject an equivalent of 3 cm into the upper 25 cm of the blue liquid. You get up in the morning and go to check on your U-tube. What do you see, and what do you expect will happen next?
– The first thing to see would be a change in the liquid level of both branches due to the 3 cm increase in volume. However, how would it affect the two branches? I assume that the starting position before injecting the light liquid is the same as we left it with 3 cm red liquid on top of 49 cm blue liquid in the left branch – and 51 cm blue liquid in the right branch. When adding an equivalent of 3 cm with a density of 0.5 into 25 cm of density 3, this would expand the volume of this part of the blue liquid to 28 cm, having an average density of 2.73, says Dr. Watson, satisfied with his answer.
– The left-hand side would stand at 54.76 cm, the right-hand level would be at 51.24 cm, and the sum is 106 cm as it should be, Dr. Watson says.
Holmes (proudly): In other words, Dr. Watson, adding 3 cm of this light liquid into the left branch produced an additional rise of its surface by 2.76 cm. Now, if something similar were to happen in the mantle – imagine what it would look like there.
Holmes: If we go back to our favourite region, the crust would be uplifted substantially and, in the process, forced to expand due to the bulging of the surface. As the crust is brittle, faults would eventually appear if the tensile strength of the crust was exceeded – and this region would be an ideal place to initiate the making of your rift valley. Because now the crust has been weakened substantially.
Holmes: And, as the terrain is uplifted, erosion sets in: transporting mass away from the region, thereby reducing its crustal mass and volume above where the uplift occurs, while creating a so-called hiatus on top of the old crust. This hiatus might cause head-scratching among later arriving exploratory geologists: “Where did the eroded mass go?” It will not be obvious with the topography they observe. As the hiatus was once located on a topographic high, the mass would have been transported FROM the later-created basin.
– You know what? My seemingly foolish mentioning of the African Rift Valley was perhaps not so foolish after all; an excited Dr. Watson interrupts Holmes.
– The rift valley is the host of several fault-hosted lakes that are slowly but surely being filled with dubious stuff and salts that could not have come from the sea, as well as more normal sediments brought in with flowing rainwater. In addition, the entire rift system is elevated more than a thousand meters above sea level! The current elevation of the entire region ought to imply that the bulk part of erosional products is heading for the sea, and not into the lakes.
– It is a somewhat funny digression, but I must tell you this, Holmes; on one of my visits to Canada, I noticed that the oily substance they call the Athabasca Tar Sands was sitting right on a hiatus. I never could understand why, – and I still can`t, but that`s just me, Watson continues.
Holmes: You are dead on Dr. Watson; I never doubted your intellect!
Holmes continues: However, we have to finish our thought experiments – in the U-tube, the light liquid molecules would sooner or later find their way to the top of the left branch using buoyancy or diffusion and evaporate, bringing the surface back to its 52 cm level.
Holmes: In our geology experiment, the rising mantle would bring the liquids to a level where they might find it easy to escape through the faults created by the uplift. Even without complete rifting, and because of loss of volume due to the escape of the light liquids – and because of the erosion caused by uplift – the surface would sink back to a lower level than it was before. On the other hand, if rifting had occurred, the mantle itself might be light enough because of its liquid content – and in real life its temperature, to rise much higher than in the oceans. The result would be a valley with a lot of faulted crustal rocks and a mantle lurking beneath the surface. Does this sound familiar?
Holmes: In both cases, a truly depressed basin is created that is capable of receiving more sedimentary mass and sinking in further. The sea might invade such a basin even if the terrain was well above sea level before and during the uplift/erosion. However, the first arriving masses into the basin would be coming from below (with the liquids) or from the surrounding region – while still being above sea level!
Holmes, getting excited: THIS is the Hippopotamus in the basin! No big sedimentary basin can be made on the continental crust without the region first being subjected to uplift by mantle movements. To make the basin, you first need a process that thins and weakens the crust in the region where a basin is later formed. Uplift and therefore erosion is the best process for this purpose. These basins will all be initially created well above sea level!
Holmes: And, this process is sooner or later bound to eject mantle fluids and dissolved elements from the mantle and crust, which makes the basin floor sink in without added mass, but rather because of mass and volume moved up from underneath! This will occur as the first water transporting terrestrial sediments arrives into the basin. The bottom of a basin will also sink in over time, as the mantle contracts due to loss of heat. Basins might sink in enough to allow the sea to invade it if a connecting path is created. Otherwise, and especially if full rifting does not occur, the basin will develop separated from the sea, in the interior of a continent. The sediments in such basins will be a mixture of substances sourced from the mantle and the initial crust, and terrestrial sediments. The hiatus will always be there to confirm the process.
Holmes continues: It is getting late and therefore: – the kind of fluids that are expelled from the mantle, and the consequences of this story for what is called basin modelling, might be a subject to discuss in another evening. Let`s go to bed, Watson.
– Not so fast Holmes, how do you get the light liquids into the deep mantle? You certainly cannot do this while I am sleeping! – And, how about another Gran Duque de Alba to celebrate, Dr. Watson exclaims.
Holmes: Elementary, Dr. Watson – SUBDUCTION! And – OK-then!
– S-U-B-D-U-C-T-I-O-N? Did you mean SEDUCTION, Dr. Watson asks, sipping to his Grand Duke.
HANS K JOHNSEN
Inspired by Arthur Conan Doyle