Timing is everything

As has been extensively discussed in a series of previous blogposts, within GeoConnect³d we are developing a structural framework. While in the initial phase of the project this framework was a rather theoretical concept, after a few months of discussions that often reached philosophical levels, things slowly began crystallizing into a concrete design. To make this leap, we felt that it was important to test the framework that we had come up with against a real dataset within the project area. The Flemish part of the Ruhr Valley Graben was selected as a pilot, meaning that we at VITO had the scoop and privilege to be the very first to try and make the structural framework actually work.

Before plunging in the challenges we encountered, the Ruhr Valley Graben (Figure 1) is to be granted a small geological introduction. The Ruhr Valley Graben (referred to as ‘RVG’ in the remainder of this blog) is situated in the Southeast of the Netherlands, the Northeast of Belgium and the West of the German state Nordrhein-Westfalen. Since the late Oligocene, the RVG manifests itself as an area of differential subsidence along a number of major, mainly NW-SE oriented, normal faults. This subsiding area was filled with a thick stack of more than 1600 m of sediments. The RVG is still active today, as is demonstrated by seismic events in the area (geomanifestations!). While the RVG as we know it now is a Cenozoic structure, it’s geological history goes back much further in time. The RVG was formed on old extensional structures of probably the Jurassic, which were in their turn inverted during the Late Cretaceous before re-evolving into extensional faults during the Cenozoic.

Figure 1: Location of the Ruhr Valley Graben (“Roerdalslenk” on the picture)

And it is the content of this last sentence, innocent as it may look in this brief geological summary, that made us scratch our heads while trying to fit the RVG into GeoConnect³d’s structural framework. This is because within a structural framework, it’s all about hierarchical classification. Following the SKOS framework, we create (large scale) fault systems in which we fit faults, faultsets, subfaults etc. From a distance it seems pretty straightforward to make the RVG a large scale fault system and then place every fault within the geographical extent of the RVG part in this system. But when you look into it into detail (and in 3D), you see a much more complex picture. As is illustrated in Figure 2, the faults in the RVG-area look quite different when you look at the level of the youngest Cenozoic as compared to the Permian/Triassic. While some of the current faults were newly formed in the Cenozoic, others have a clear geological link to older faults (from which they were inherited). Some of the older (Jurassic or Late Cretaceous) faults were then again not reactivated during the Cenozoic. The same set of faults also displayed different kinematics during different periods, from reverse during Late Cretaceous compression to normal during Cenozoic extension.

Figure 2: Belgian faults in and around the Ruhr Valley Graben. Left: top Cenozoic, Right: base Permian/Triassic

Consequently, you cannot really understand the geological picture of an area by only describing structures hierarchically, in terms of larger and smaller. You need the timing of the fault activity as well. Hence, we felt like it was essential that our structural framework would fully include the aspect of time. Not just by adding some reference to the timing in the definition field in a vocabulary file, but as a proper attribute on which you can query. And so we did. We thereby immediately noticed that this was difficult and complicated and time consuming. But also that it made us think of the faults in another way, that we started grouping them differently than we did before, and that we were acquiring a great deal of new insights in faults we thought to know already quite well. And when discussing our findings with TNO as was explained in a previous blog post, we found that they encountered the same challenges and that there is room within HIKE’s Fault database to integrate this concept, which can only be beneficial to everyone.

In an ideal world, this timing aspect would be firmly embedded in the GeoConnect³d structural framework as a fixed set of tectonic phases which are referenced to by everyone within the project. And while this last part might be a bridge too far for GeoConnect³d, we hope that this blog encourages all project partners to fully take up the aspect of time when drafting their local structural frameworks. Because after all, timing is everything.

Katrijn Dirix and Jef Deckers
VITO, Vlaams Instituut voor Technologisch Onderzoek – Belgium

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