If you stumbled upon this post, you are forgiven to find the next phrases a little quaint. That is because it took three posts (part I, part II and part III) to come to the point where the structural framework suddenly realises that it is still very little. It therefore starts asking: what can I be when I grow up? And like any nice person, you answer: anything you like, sweet little thing.

And like always when we are trying to be nice, we lie. We say ‘anything’, but really mean ‘useful, reliable, and easy going’. Hopefully the structural framework is going to be ok with that, because GeoConnect³d is a parent that already has planned its child’s career.

So far on our wish list we find two important elements. The framework needs to exist of structures that are related, but preferably not in a complicated way that reminds us too much of structural geology, or mathematics. Also, we want it to look like a collection of geological units. Sounds simple, right? Well, anything too straightforward is to be regarded as suspicious.

Geological units are big chunks of the Earth’s crust, and therefore quite different from the faults that we mostly have been discussing as the building blocks of our framework. Try to imagine what is left after leaving 1 trillion cubic kilometres of molten rock spinning round for 4.5 billion years: no way that is going to end up as something nice and organised. It is not a coincidence that geologists prefer hammering rocks instead of looking at them, and seem to have an urge to get dirty whenever they go outside. You become what you study, you become messy.

Now imagine that you are looking at geological units on a map. You may think that it is similar to a stone wall, a collection of more and less interesting rocks with some cement in between to keep them together. Good thinking, but wrong. Examples of geological units are a terrane, a fault block, or a basin. These look like different entities because they acted as separate units during some part of geological history.

That may sound as a satisfying definition of a geological unit, but the most important question is: why are they different from neighbouring units? And why was the geological history different from one unit to the other? The reason is that units have one or more limits, and it is hard to overestimate the importance of limits.

Those limits can obviously be faults. A good example is a graben, which is by definition a fault bounded area of subsidence. If fault activity is reversed, it becomes a horst, a fault bounded area of uplift. In these examples, the geological units graben and horst are blocks that are being defined by the faults, or the structural framework. Defined should be taken very literally here: the graben exists only if its bounding faults exist and are active.

But faults are just some of the structures of interest. A terrane, for example, is a piece of continental crust that was once separated, all by itself, surrounded by nothing but ocean. Now it is bounded by suture zones, the lines where the oceanic crust was subducted before the terranes crashed into each other. Well, very slowly crashed into each other, we are talking geology. Such suture zones are not really faults, but they certainly are structures.

And then there are the sedimentary basins. Do these have limits? Well, they do if you think in 3D. A basin is typically defined by a basal unconformity, marking the start of subsidence and sedimentation cycles that are typical for a certain basin.

So, we are calling unconformities structures now? Yes, but only if they define geological units. The goal of a structural framework is to structure things, to tie them together. This means that anything that does this will be treated as a structure in our framework.

So far so good, but this is only the beginning. We have a clearer idea of what the structure framework will be: a network of limits and the different geological units delimited by them (see the figure below). Now it is time to start combining the geological information of our areas of interest – covering limits and units from pan-European to local scales – into one coherent model. Bringing up a structural framework is a tiring task, and it isn’t even going through puberty yet.

Kris Piessens and Renata Barros
Geological Survey of Belgium

Reference:

Jähne, F. (2014): Geology of Europe. In: Reimann, C., Birke, M., Demetriades, A., Filzmoser, P. & O’Connor, P. [Eds] Chemistry of Europe’s agricultural soils – Part B: General background information and further analysis of the GEMAS data set. Geologisches Jahrbuch (Reihe B 103), Schweizerbarth, Hannover, 352 pp.

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